There are loads of different ways to do abs exercises and I guess its finding the ones that work best for you - and that exercise the upper, lower and oblique muscles.
For crunches, its absolutely fine to put your feet under the sofa, but as Jess says, as you are not coming all the way up - you shouldn't have to. Best to vary the tempo - all should be controlled though. Vary between straight up and down, then up for the count of two (half way, then all the way) then down for count of two.
There should always be a gap between your chin and your chest - imagine there is a grapefruit sitting in there!
Best to do them on a solid surface to support your back- but with a yoga mat or something under you- a folded up towel or carpet should also be fine.
The plank works the abs a treat but its quite hard. Go into the position of a press-up but keep your body relatively flat - then hold the position for about 30secs using your abdominals to keep your body off the floor - this one doesn't hurt you back and neck as much.
Happy exercising....