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. But even if you have a cable-modem connection, you might consider using powerline networking technology to help relocate your AP.
The current generation of powerline products based on HomePlug AV or DS2's "200 Mbps" can provide an average throughput of around 50 Mbps. While this is half of what you would get from 100 Mbps Ethernet, it can be perfectly fine for making the connection between a broadband modem and wireless router.
All you need to do is buy a pair of adapters such as the Zyxel PLA-400 [reviewed], Linksys PLE200, or Netgear HDX101, plug them into the wall and plug Ethernet cables from the modem and router into the adapters. At around $70 each, this solution can be a bit expensive. But it's a lot easier than snaking CAT 5e cable through walls!
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 later, 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.
Since you now know how to perform a site survey, make sure to use that tool to confirm that your AP's new location actually is better. You might find that you've solved one coverage problem, only to create another!
Once you've exhausted the no-cost option of just moving things around, the next steps involve spending money. The trick here is to spend wisely and get the maximum bang for your buck.