BMI (Body Mass Index) will indeed give a false impresion of weight/body composition as it's a simple height/weight ratio calculation (weight in kg X height in meters squared). The best body composition calculations are the electronic ones (e.g. bodystat 1500) which actually tell you your body fat percentage (the important reading) as well as lean body mass, water content (it can tell if you're dehydrated etc.) and so on:)
Skin fold calipers are only really accurate if you use a decent, medical grade set (not the cheap plasic ones with chinese springs as sold on ebay) and are used by someone who knows how to take the proper measurements and measures from the correct places - the more accurate calculations use 6 measuring sites (biceps, triceps, sub-scapular, supra-iliac, pectoral and femoral points).
The best general measure is the wobble test - stand naked in front of a full-length mirror and jump up and down to see what/how much moves:)
The main thing is to NOT get hung up on weight - it's all about body fat NOT overall weight:d