its always hard to give a definite answer without being there, howerver there are a few general rules of thumb
1, if possible try to use channel 6, generaly speaking most antennas will be tuned to this mid point frequency, also a lot of the channels have a small amount of overlap with their neighbourghing channel, with the exception of channel 1, 11 and 6.
2, carry out a wirless audit and make sure that no other nearby set up is using the same channel, this can cause interference.
3, keep all other wirless electrical devices as far away as possible. This is also true with microwave ovens as these cook your food using almost the same frequency as your wireless router
4 download netstumbler and play around with it. use it to check signal strength, check for other networks etc. try moving things about one at a time (ie objects that may interfere, network card, router etc) then go back and see if there any improvment.
hope this helps
si