One of the limitations of a map and compass is that it is very easy to walk off a map's boundaries... in which case all you have is a compass. You really should have a full set of maps of an area, even if you don't "think" you're going to need all of them.

A GPS isn't perfect either... you still need coordinates if you want to get from point A to point B, and a GPS won't tell you if there are impediments in the way (like cliffs/dropoffs, rivers, etc.).

Topo maps give way more information than just a trail. You can get lost with a map and compass, sure, but you can get information from a map that you can't get from a GPS. Map and GPS is a good combo, but you could run out of batteries, drop the GPS, have an electronic malfunction, fail to pick up a sattelite, etc etc etc...

It really is very difficult to judge distance to a landmark in the field, and when you are hiking in heavily wooded areas, you can't triangulate unless you can find 2 very clear landmarks, like radio towers or Fire Observation towers, etc. Trails certainly are not well represented -- location wise -- on topo maps, so you can't rely solely on the direction a trail is actually going versus the direction is it "supposed" to go. The best way to keep from getting lost with a map and compass is to familiarize yourself as much as possible with all information you can gleen about a trail/route/area from topo maps, aerial views (GOOGLE Earth), trail guides, etc. etc.

No magic answers, I'm sorry to say. We usually have Aerial ops to compliment our ground crews.

MNS