If you are the hotspot provider, signal strength can be increased in numerous ways: increasing the transmit power of your AP(s), relocating AP(s) to provide better coverage of the intended area, adding a high-gain directional antenna to better focus each AP's radiated power (benefit depends on shape of intended area), or adding more APs to provide higher-density coverage. Because hotspot providers need to accommodate as many different clients as possible, other options like using G-only mode or pre-N products are not really helpful here.
If you are the hotspot user, there's nothing you can do to change the hotspot's output. But there are a few things you can do to improve your own client's reception. The easy first step is always to relocate -- use your Wi-Fi adapter's signal strength meter to determine where coverage is better, avoiding obvious RF obstacles like fountains and heavy stone columns that tend to create "dead spots." The next easy step is to change the way that you use your client -- for example, turn your laptop 90 degrees and clear the space around your Wi-Fi adapter (including the portion that's actually inside your laptop). If you consistently encounter poor signal strength at every hotspot, consider buying another Wi-Fi adapter with better transmit power and receive sensitivity. Some hardware comparisons can be found on the Seattle Wireless.net Web site.
This was first published in December 2006