Cheap Ways To Improve Your Car
Here's a few ways I have found to improve your car for a cheap price and is also fairly easy to do. Maybe most of this info is useless but I hope you can find a few good points. If you look hard enough, there are lots of cheap parts sold privately or at the wreckers. I also take no responsibility if you screw up.
Under $100
Floor mats ($40)-Putting in black floor mats made the floors look much better and cleaner for my car (which has a dark grey interior). Also, they are easy to clean since all you have to do is vacuum it once in a while. It also helps the original carpet underneath stay new looking.
Racing pedals ($40)-It gives the driver's area a nice touch even though they actually do nothing for me performance wise.....
Redo the interior ($25)-I redid the whole upper half of the interior with black velvet (not the crush lowrider type) and the factory speaker grills. It took about half a day to cut the pieces and glue and staple (on the sides where you can't see) it on. Getting the ceiling panel out was a pain in the ass but it definelty looks a lot better than the original light grey upholstery.
Side marker lights ($40)-I picked up some universal clear side marker lights which require only one round hole (unlike some of the other ones that need weird shapes cut out to fit). You can use paint or a permanent felt pen to color the bulb so it shine what ever color you like. I noticed using a felt and making blue stripes makes it look a bit like those euro lights.
Clear taillight and turn signal lights (about $30)-I made them myself with materials you can find at any building supply store. The turn signals took 1/2 hour while the rear tailights took half a day since it involve lots of grinding and bending. Please check out the section on how to make them if you want to try. You can also rearrange the wiring so the top lights come on for brakes and bottom ones for turn signals. I left mine the way it was since it's too much hassle.
Neon lights (about $40)-You can find them at most customizing shops. I got mine at a Taiwanese car customizing shop for $40 for a 1' bar. Some places charge over $80 but I thought that was a rip off. I used mine for week and that was it though....
Paint visible parts ($5)- Buy a some rust resistant paint and paint brake drums and calipers. If you know how to remove those parts, it makes it a lot easier and you can do a better job. You don't want painted brake discs. It makes it look alot better especially if you have mags which make those parts visible. Painting engine parts helps a lot too. Make sure you use proper paint or it will start peeling and make it look like a piece of s--t..
Chrome mud guards ($25 for four)-Made my car look a lot better since my car is silver and has polished wheels. It's very cheap and easy to put on and protects my paint from flying rocks and debris.
Decals (varies)-Putting decals on in a tasteful manner does a lot for the exterior. Putting on lots of huge decals all over the place makes it look stupid and cheap (unless your car is a sponsored drag car or something). I don't like shaved emblems so I bought a '98 "Accord" decal when I got my car painted. The newer decals look much better than the old ones. Only problem is that Honda charges quite a bit for a stupid piece of plastic (about $40) but I thought it was worth it.
K&N Filter ($50)-These are fairly cheap if you consider the fact that they are re-useable. The recharging kit is cheap and the filter does improve power (not too much for the 2.0L engines though) so it's something any car owner should consider getting.
Fog lamps (Starts at $40)-Depending on what you need them for, the price really varies. The cheap ones are just for looks rather than for improved vision. If you spend a little more for good ones like PIAA ($200), it will help night vision alot. Since I am cheap, I just bought some cheap ones for about $80. Cars do look better with fog lights and installation is very easy.
Spoiler (varies)-Mine was cheap but it looks kind of cheap too. But I couldn't say no to a free spoiler. This inproves the look of any car and you might be able to find ones for sale privately in the local paper. Factory ones cost over $300 while aftermarket ones start at about $150. I've found many used ones in perfect shape (in the paper and at the wreckers) for $75. Just make sure you don't over do it. Putting on a whale tail on a 83 Accord doesn't look sporty.
Tune up the engine (about $75)-Picking up some high quality spark plugs (like Splitfires or NGK's), good distributor cap and rotors, and some racing plug wires will improve power and fuel economy. For my car, it made a difference of about 8% for the fuel economy after changing those things right after buying my new car. It also runs alot quieter and smoother too.
Other (Cheap)-Polish the whole interior with Armor All or some kind of protectant. It makes a huge difference if you do it nicely. You can do the same for plastic engine compartment parts. You can also pick up some new vaccum lines and hoses to replace the old faded stock ones. You can buy them in black or go to a good parts shp and get the color of your choice.

Under $200

Phantom Top (varies)-If you find the right combination of colors, this can make the car look really nice. I've seen some ugly ones in the past and I don't want to do it to my car since I'll probably want to take it off after a while. Also, make sure to get it professionally done since cheap phantom tops tend to form bubbles or deform sometimes after going through some rough weather. Cheaper's not always better.
Chrome fender trim ($150)- I guess this is one of those things that depend on the owner. I'm not a big fan of it but it does have another function other than looks. It can be used to cover up rust bubbles around the wheel well. If you don't want to fix the rust, you might want to consider getting some especially if you find some in the paper where it usually sells for $75 for a set of four.
Exhaust (varies)- You can upgrade your exhaust for very cheap. You could install a glasspack muffer sold at local auto stores for about $30. Just make sure you get the right adapter piece to conect your existing pipe to the new muffler. You can also have one custom made at places like Midas where they will find a system to suit your budget. If you want to go even better, ones can be bought through customizing shops. But make sure to ask questions first especially if you are buying it to improve power/torque. Some mufflers are strictly for looks and sound and the added weight of some of the bigger ones can cost you overall performance.
Header (varies)-These definetly improve horsepower but it's not that cheap. If your lucky, you might be able to find ones in the paper for about $125. New ones could hit you in the $500 range but if you are serious about power, this is a must. I got a DC Sports header and I noticed a pretty decent increase in power and much better fuel economy (12.5km/L or 30 MPG) after all my mods. It used to be about 11.5km/L or 27MPG

Things you should consider even though it's expensive
Paint the car (varies)-Don't laugh. I got my car painted at Maaco for $400. I didn't want to spend too much money so I went to them. I got them to throw in lots of metallic and clear coat and I was surprised at how good it looked. But, if you have the money, it's definetly better to go to a certified paint shop. They give long term warranties which is the biggest plus and the paint quality is better.  Also, if you can get really good paint at the auto store, you can get Maaco to knock off a certain amount off their paint packages (but it might depend on the branch) and get them to paint the car using your paint. Eg. Buy paint for $150, go to Maaco, get the cheap package ($300) they knock off $100 (what the branch I went to said) so you end up with a good paint job for under $400. I had trouble with their paint since my body kit was made of urethane so the paint was chipping off after half a year. They fixed everything for $100 which was nice on their part.
Body kit ($1200)-This costs a lot and you have to get it painted but I think it is definetly worth it since it really makes your car stand out from the other ones that are just lowered. I get people pulling up next to me at intersections commenting on how my car looks nice even though there are much nicer ones out there which really feels good. The surprising thing is, I even get adults and kids saying the same thing.
Mags (usually about $1500 for a package deal with tires)-Depending on the ones you get the price really varies.