Depending on your setup, you'll want to go with either an omnidirectional antenna that scatters the signal throughout your home or a directional antenna if most of the devices that are in need for good throughput are in one room. Probably the best and most extensive guide for replacing antennas is Binary Wolf's.
4. Figure out the best spot for your router
Use a Wi-Fi heatmapping tool to measure the impact of distance, frequency changes and building structures on signal strength. Two tools that are great for this job are NetSpot for Mac and Heatmapper for Windows. Both tools allow you to track Wi-Fi coverage in your office or home. In this example, we're going to show you how NetSpot works: Once you've installed the software, type in a new "Site Survey" name and hit "Blank Map". You can also select a floor plan of your home or office and get an exact map. If you're more creative, I suggest you select the "Draw Map" feature and start drawing your own floor plan. Next, define the scale by determining the exact distance between two spots. Hit "Let's get started" and just walk around. Click the spot on the floor plan that you're currently standing in:
Obviously, the more points you scan, the more exact your Wi-Fi heatmap. Once you're done, you end up with a map that shows you not just the signal strength but also the throughput of your Wi-Fi network.
5. Varying CPU frequencies and their effect on wireless signals
Your computer's motherboard is also working in the "Gigahertz" spectrum. That "noise" is being picked up by your built-in Wi-Fi transmitter. Unfortunately, the higher that noise is, the more likely it is for your wireless adapter to lower bandwidth automatically (by lowering the link-rate and avoiding frequency interferences). As CPUs these days clock dynamically, the Wi-Fi adapter needs to constantly adapt the link rate which not only causes a variation in Mbps but may also be the cause for dropped connections. Especially on laptops, the Wi-Fi adapter is often built close to the memory and CPU bus, which is a major source for problems.
Of course, this all depends on the design of your Wi-Fi adapter, but if these symptoms sound familiar you might solve this issue by getting an external adapter. Some of these adapters, such as my Linksys adapter, even have a little stand that's connected via a long USB cable. Putting that kind of space between the Wi-Fi adapter and your CPUs noise is likely to help a lot. Of course, that's not too handy when you're traveling, but at home it's a viable option. Typical Wi-Fi adapters such as the Linksys AE2500 (802.11n dualband) or the MSI US310EX will set you back between $20 and $40 and they're worth every penny.
6. Firmware or driver issues
An easy, yet often forgotten piece of advice: Make sure that your router's firmware is up-to-date -- especially if you've purchased a new one. Expect bandwidth, feature set and resiliency to signals to increase with the first few firmware updates. (My Linksys router could only deliver full bandwidth to my living room after the upgrade.)
Also make sure that the Wi-Fi adapter (either external or built-in) is always up-to-date. Dropouts, standby issues, low performance may be gone in the next 0.1 release of your adapter's drivers. Although the frequent driver delivery via Windows Update has gotten better in recent years, it rarely fetches you the latest and the greatest drivers. Instead, do this...