After cooking, make sure you let your joint or poultry stand for at least 20 minutes, so the meat 'relaxes' and the juices flow through the joint. You'd be surprised how many people attack their roast as soon as it comes out of the oven, and then wonder why it's dry and chewy.
If you're thinking of using an electric knife, here's my advice: Please don't.
Ideally, you need a long, sharp knife. A serrated blade works quite well if you'd rather not be sharpening all the time.
Use the entire length of the knife to carve, cutting against the grain to slice up the meat fibres.
If you use a fork to hold the meat in place, press lightly. If you stick the fork hard into the joint, everything will collapse and you'll squeeze those sumptuous juices out. Ideally, you should only use a fork to move the sliced meat.