When you're deploying multiple APs you also want to turn down the broadcast power on them. If the signal of multiple APs overlap too much, clients won't roam onto the next AP in time.
Also don't be afraid to hire someone to do a wireless survey - or do it yourself. Someone will walk around with a laptop, and try to find wifi blackspots/hotspots, and can recommend adjustments to AP power and/or placement.
Yep, and disable the lowest data rates, particularly if you have decent coverage. The AP is forced to send certain types of traffic at the lowest available data rate so everyone can hear it so you save a lot of airtime on that traffic but also clients will be more likely to roam because they can't "stick" to a far away AP at a really low data rate even if they wanted to.
Also don't be afraid to hire someone to do a wireless survey - or do it yourself. Someone will walk around with a laptop, and try to find wifi blackspots/hotspots, and can recommend adjustments to AP power and/or placement.