You can try an antenna upgrade -- I did that last summer and it seemed to help with my patio coverage (router is in second floor office) but earlier this year, we started having issues with signal downstairs and I suspect it was due to interference as I am starting to see more and more wireless networks in this area.
So, I bought the
Amped Wireless SR10000 and replaced the DAP-1522 wireless bridge that I had in my downstairs entertainment center. The SR10000 serves as both a bridge AND wireless repeater. Some comments:
1. If you get one, make sure you upgrade the firmware the very first thing.
2. Wireless signal is definitely improved throughout the first floor and even onto my patio.
3. Here is the kicker, however -- it will NOT repeat your existing SSID; you have to make a new SSID for what they call the "extended" network. Intuitively that makes sense when you think about it since you're bridging back to the original wireless router, but I didn't think about that beforehand and was a little disappointed. At the end of the day, I just add "A" to the end of my original SSID and used the same WPA preshared key and it wasn't bad at all.
The SR10000 is right in the middle of their lineup; they have 2 models below it and 2 above it. My advice is to check the link above and if you want to try it, order from Amazon so you can return it if you don't like it.