CJoseph has the best plan for you, but in terms of what to think about, here's how I'd approach it:
If you know you have a poor spot, look to see if you have interference in that location, and see how many access points you have covering the area.
Remove or mitigate external interference where you can.
If you have several APs covering a spot, consider turning DOWN your power to reduce the co-channel interference - I know it sounds wrong to turn down the power, but I have a warehouse which went from nearly unusable at 18 and 21dB to nearly trouble free at 12 and 15 dB.
If only 1 AP then you'll want to look at adding (which I know isn't in the plan right now, but note the area as one of your first priorities)
QoS will only really help a small number of users with very specific issues.
Can you elaborate on the issues your users are reporting?