I play a couple of flat hard serves who regularly hit 110 mph. The easiest serve to return is a flat serve hit straight at your stikezone. There are 2 effective returns for these serves:
1) The block return. When played well, it keeps low and uses the pace of the serve. You need to practice moving forward and finishing the point
2) The short-arm drive return. More offensive and high risk but redirects most of the pace of the serve.. There is little you can do about it if placed well.
When serving, try and anticipate the direction of the return based on the type and direction of the serve, and start your initial movement accordingly. For example if you serve a good slider out wide, its most likely to be returned crosscourt.
Also when serving, focus more on the angles, makes a big difference if you serve flat. Try adding a bit of overspin to it, will help you get the extra marging you need to get those angles.
K Six One Tour 90/Volkl PB10 Mid,
Tecnifiber Multifeel/Luxillon Alu Power (50/47 lbs)