I'm pretty handy at DIY but up until a few weeks ago I had never got involved in plasterboard. My brother in law came over and after some tips i've boarded several walls (doing total refurb on a old house!!).
This is what I have found;
Make sure the boards are level, buy a long spirit level for this.
Measure the size, mark the boards on the face side with a pen/pencil, use a stanley knife to score a line in the plaster, turn it over and give it a tap, the plasterboard will snap along the scored line. On the reverse side run a knife along the score and the plasterboard shoul come apart neatly. For small areas cut with a stnley knife by repeatedly scoring on the line until it seperates.
If there are battens where you are fitting the boards, (replacing lathe and plaste for example) screw directly into the battens using plasterboard screws, taking care to ensure you butt up joints on a batten for stability.
Get as tight as possible on the joints but a couple of mm gap in places will be OK as before plastering you will cover the joints with scrim tape.
If fixing to brick/block mix up plasterboad adhesive using an electric mixer (or stirrer attached to a decent wattage drill, I bought a £8 stirrer and 800W drilkl from Wickes which is fine) put a small amount of water in a bucket add some advesive whisk gradually adding more adhesive until it resembles butter. Dab spots of advesive on to back of boards about every 12". Offer up to the wall, push board into place and using a decent size block of wood, tap it square. If securing to breeze block you may also be able to secure with screws as well.
Use small offcuts on the floor to stand the wallboards on when fitting, this stops them resting on the floorboards and will prevent cracking of plaster if floorboards are moved. (Once secured in place remove).
Here are a couple of web sitres that may be of use
http://radio.weblogs.com/0136203/2005/02/Plasterboarding-techniques-hints-and-tips.html
http://www.diyfixit.co.uk/nflash/building/Plastering/plastering.htm
I found doing it relatively easy, with everything patience and being accurate is everything. Wherever possible use long lengths of board (avoid the urge to use up the little bits) as it helps keep things square
Good luck, hope its of use
Andy