This is probably not complete - I guess I can always come back and add more - but I've tried to make it chronological at least, so these are the things I think you need to be worrying about when you think about your roast.
- Buy Good Quality Meat
This has to be top of the list. More than absolutely everything else this is the number one factor that will make or break your finished meal, as without a good piece of meat to start with you're never going to have a good finished product. Don't confuse good with expensive either - some of the most fab cuts are wonderfully cheap (e.g. shin of beef) - you just need to know how to cook them. - Don't Roast it Straight from Cold
Roasts are almost always big pieces of meat, and if you take them out of the fridge and put them straight in the oven then meat in the middle of the joint is going to heat up much slower than that on the outside. Sometimes this can be a benefit - perhaps you want to keep the middle of a joint of beef very rare - but personally I always find it best to take the meat out at least an hour (sometimes longer - depends on the meat) and leave it to get up to room temperature. Don't fret too much about it going bad - an hour or so won't hurt (though don't leave it anywhere warm like on a radiator!) and you'll get a better end product. - Season Well Before Roasting
Seasoning is always critical, and this applies just as much to roasts as anything else. You can be quite generous with the salt & pepper. Do make sure you do the inside of poultry and rolled pieces if you can, but don't over do it on pork crackling or it'll be over salted. Also, don't salt early - it'll start to leech juices out of the meat. Do it just before the meat goes into the oven. - Know Your Oven!
You don't necessarily need a great oven for a great roast, but you do need to know what you've got. Some ovens create temperatures that are widely different to what they display or what their controls suggest, so if you're in any doubt over your own oven check it with an oven thermometer. You never know, all those apparent failures may have been the oven's fault and not yours at all! - Hot Then Cool
Many cooks use this technique, and though I deviate from it quite frequently (with slow roasts and spit roasting in particular) it's still the one I do most often. Basically you start the roast off at a high temperature for a short time to start crisping up the outside and force heat into the meat, and then turn it down for the rest of cooking so you don't burn the outisde and the heat gradually permeates through and cooks evenly. - It's Done When It's Done
At the end of the day a roast is ready when it's ready, all you can do with timing and temperatures is try and get some sort of reliable guide as to when. There are lots of ways of checking when meat is ready, a thermometer apparently being the best (though personally I've never used one). For chicken and pork it's the juices running clear when you pierce the thickest piece of meat, for lamb and beef it's pretty much whenever it's gets to the stage you like it at! All I'm saying here is don't just assume it's ready- at least don't be a slave to the timing. - Rest the Meat
Resting is crucial. In fact if I was ordering these by priority then this would be number two after the quality of the meat. Resting allows the meat to 'relax', and ensures that the juices flow back from their excited states back into the fibres where you want them, creating a tender juicy mouthful. All joints could do with 20 minutes or so - large joints even more. Do make sure that you rest somewhere warm - not hot, not cold, but warm. Still in the oven is OK, provided the door is open a little to let some heat out, but you'll need the pan it was roasting on to ... - Deglaze for Gravy
Gravy has been many peoples bugbear, mine included, but no longer. Here's what I do. Once the meat is out of the pan and resting somewhere get rid of most of the fat from the pan (if it's good fat, like beef, then keep it aside for other cooking) and put the pan back on the heat on top of the stove. Add some liquid, and scrape up all the good stuff in the pan into the liquid, and reduce till you get to the consistency you like. That's it. If you like a thick gravy leave a little fat in the pan and stir some flour into it and cook it a little before you add the liquid (a little at a time). If you want to include some other flavours use wine, port, stock, water from vegetables, anything really in place of plain old water, or stir in some mustard, fruit jelly, whatever you fancy, but basic gravy is really quite straightforward. - Serve on Warm Plates/Dishes
Nobody wants to eat food so hot they get burnt, but the problem with roasts is often the opposite - what with resting and all the other mucking about the food can be too cool by the time it hits the table. To get around this you really do need to warm your plates beforehand, and by warm I actually mean quite hot. You don't want the meat to sizzle when you put it on (and believe me I've done that a few times!) but you do want them practically too hot to handle.