You could get an after market antenna that has a higher transmit power to it, or you could get a repeater, but other than that, the only real solution is to move the router or the console.
I'm using the wireless conection of my PS3 but only getting about 45% signal strength. Must be the location under the TV and several walls between the router and the PS3. Wife (upstairs overtop of the router) gets much better with her access point/desktop.
Can I use a Linksys access point with the PS3 to improve the signal strength? USB connection?