Location, location, location
The least-expensive thing you can do to make things better in WLAN-land is locate your Access Point or wireless Router as close as possible to the area where you need the best wireless connection. This may be easier said than done, however, especially if you're tied to a specific spot because of where your cable or DSL modem line enters your home or office. Of the two, a DSL-based connection is probably easier to move, since you may already have other phone jacks that are tied to the same phone line.
Once you've picked your location, the following rules of thumb will help with the final AP placement:
1) Higher is better than lower
2) On top (of a cabinet, bookshelf, desk hutch) is better than inside
3) Away from large metal objects (filing cabinets, steel shelving, etc.) is better than near
If you follow the simple practice of trying to "see" (remember the light bulb and translucent-walls analogy) your access point from wherever you want to use a wireless client, you may quickly find some obvious problems. You might also find some not-so-obvious ones too, like the aquarium that one home networker realized was killing his WLAN connection (water weakens high-frequency radio waves). Watch out, too, for utility rooms and attic spaces that might be lined with foil-backed insulation or metal firewalls or doors. Trying to figure out why you can't get a good signal out on your deck? Aluminium siding or window-screens could be the culprit!
The same goes for the locations where you use your wireless clients. You've probably noticed that you get a better signal when you orient your notebook computer in a certain direction, or move to another part of a room. I'll talk more about client-based solutions shortly, but don't overlook moving some furniture around or even moving your favorite chair, if that's where you'll be doing most of your wireless computing.