+1 to the thermapen, cry once but enjoy for long time (unless you do like me and melt it and then cry three times - once for the initial buy, once when you melt it but sitting it on a hold pan lid and once when you rebuy a new one).
The temperature situation is a smidge more complicated than thus far stated. I had to bust out my copy of McGee's "On Food and Cooking" to cross check here, but I'm still paraphrasing and simplifying wildly so while I believe this is accurate I may be oversimplifying excessively, misleading through ommission or mistating in some way (aka if I'm wrong its me, not McGee..) McGee is highly recommended if you are a food nerd, like useful reference books a lot or just really want to know why something works the way it does - or indeed .. didn't work as you'd hoped
http://www.amazon.com/dp/0684800012 If McGee is to geeky I'd recommend "cookwise"
http://www.amazon.com/CookWise-Successful-Cooking-Secrets-Revealed/dp/0688102298 If you like books half as much as I do.. I'd recommend getting both.
The short version:
If you hold meat at around 120F you get a "slow aging" effect when one protein (myosin) coagulates and pushes out some of the moisture. This is advantageous for some meats (like prime or tri-tip) as it condenses the flavor and enhances the texture.
You need to get the meat to ~160F and hold it for a while to actually break down the collagen into gelatin, this takes both correct temperature and time (time depends on amount and type of collagen - for something with a lot of it like brisket it can take multiple hours).
Above 140F the meats cells shrink (simplifying wildly here) and push out a lot of moisture making the meat drier. This happens even when wet cooking which is why braised meat can be fantastic while boiled meat feels dry and chewy even though it was wet cooked.
So there is a problem here, higher temperatures == more tender but drier meat. We like tender, we like juicy.. what to do!?
Cooling the meat after cooking allows some of the proteins that got all shrunk up to relax and will allow the cells to re-absorb moisture. In order for this to work properly you have to let the meat get back into the sub 140F range before you cut it (hence the famous "resting a steak" - but its useful to know an actual temperature target). As an aside you can quickly "calibrate" your finger tips to within ~5F at least in the 100-170F range by feeling things and then checking with a thermometer.. I found that out when brewing a bunch of batches of beer in sequence, amazing what the body can do (and that's why practiced chefs don't use thermometers - the rest of us are better of with good tools because we don't do it 10 hours a day).
So for tender cuts with low fat (like say prime rib or stir fry) you'd never want to take the bulk of the meat much above 140F because you don't need to break down much/any collagen and a short rest afterwards will help them re-absorb the moisture (a rest is impractical with stir fry or similar but the temperature issues remain).
For tougher cuts manage your temperatures carefully. Get them up to collage breakdown (my new band name I think - as soon as I learn to play an instrument past three songs) temperature and hold them there and then allow them to rest afterwards to below the re-absorption temperature.
For the more difficult cuts you can also help things along a little with some "helpers". Acids accelerate collagen breakdown so some vinegar or tomato or similar in a marinade will help that; alternatively you can use tenderizers like papaya or mango (or commercial tenderizers which are usually extracts of papaya) but you have to be really careful with them or the meat ends up mushy. Brining the meat reduces moisture loss (I'll skip the cellular hydrostatic pressure details
) and helps keep it juicy even at higher temps.
As an aside browning or searing meat does not (contrary to popular myth) retain or hold in moisture. It does add delicious flavors and kills surface bacteria so its worth doing but there are a few things to be aware of. Sear hot; you just want to cook the surface, if the temperature gets to low it will take to long to get the surface brown and the inside temperature of the meat will rise making it dry (and probably tough) so a hot pan that holds heat (cast iron, best way to go..) will do a better job. The browning action itself due to the maillard reaction, this is a reaction of amino acids and sugars and requires a lot of heat (say over 230F) which is only achievable in a dry(ish) environment (otherwise the boiling temperature of the surface moisture limits the temperature to that of boiling water ~212 or less). So if you've brined or marinaded meat and want to brown it, pat it dry and then let it sit out (covered in a towel) on a rack for an hour or two to let the surface dry off. This is true of small/tender cuts meats as well. Nothing ruins a stir fry like boiling it because your temp went to low and cooked all the moisture out of the meat or there was residual surface moisture that dropped the interface temperature (which then..), unrecoverable and dinner is gonna be chewy and dry.
Different meats of course have slightly different temperature sweet spots but the temps here are close enough for most purposes.