I am a ski mountaineering novice, but everything that I have read says wall-to-wall skins are the way to go, and my (limited) experience bears that out. Like Crudfish said, you want to trim them so that the edges and a couple of mm are exposed, so buy a width based on the ski's maximum tip dimension (about 5-10cm less should work). Instructions are on the Black Diamond website (www.bdel.com)
. I use the Black Diamond Ascension skins. They make 2 kinds, the original Ascensions and GlideLites. The latter are more flexible and easier to stuff in your pack or pocket, but the stiffer ones are easier to handle if it's windy. The pile is the same on both, as far as I can tell. They got top reviews in Couloir magazine's tests. Most people seem to use either the BD's or the G3's. Colltex mohair skins have better glide and are mostly used by rando racer types and are not as durable.
Fixation systems: all of them have glue on the back. Don't even think of getting those skins without glue (I think Voile made some strap on things in the past, but I have heard they were horrible to use). Most (like the BD's, G3's) have a loop at the tip that you hook over the tip of the ski first then stick the skin on from tip to tail. The tail fixation device is either an elastomer adjustable thing with a hook for the tail (used on the G3's and called the STS tail on the BD's) or a metal camming gizmo (ClipFix on BD). I have used both and they are both easy to use and work well, but the ClipFix type enables you to adjust for different length skis more easily. Thats what I ended up buying, but mostly because they were on sale.
You may have problems with the tail if you have twin tips (one reason they are less popular in the BC) and see Lou Dawson's modification on www.wildsnow.com
for a solution for pesky tails that keep coming loose (not a problem I have had). Some people cut a notch in the tail with a Dremel tool to keep the tail fixation device from slipping.