I restarted bowling spin about 2 years ago and here's my advice:-
1. Concentrate on being accurate above everything else - batsman get out if you bowl straight enough times
2. Don't get too hung up by flight or turn, a bit of both helps but you don't need Swann like loop or warne like turn to confuse a batsman. Just enough loop to not make it medium pace and enough turn off the straight now and again to keep him guessing.
3. Concentrate on "completing" your action. ie. bowling through almost like a medium pacer
4. Get a 2 coloured ball to see your seam position on release
5. Don't bowl too slow (from a bit of a dart merchant I admit), see a lot of guys float the ball up with no bite and just allow batsman to pick a spot. Bowl at a decent pace.
Don't agree with the first or the last points there Nick. As a spinner, you are there to turn the ball above else, surely? I've been bowling spin for 6 or 7 years and I've found that the more I try to complete my action and turn the ball (both are linked in my opinion) the more wickets I get. As for accuracy, as long as you look where you want to pitch the ball then I find that it goes somewhere near there (not sure how to explain that better! Basically I worked on spinning the ball first, and then accuracy came as a by-product). Drift/loop will come when you get the ball spinning nicely through the air, but usually comes easier to taller people. Drills for dip - put a string across the nets (tying each end to each side of the net) and try to bowl the ball so that it goes over the string but still lands on a good length.
As for bowling too quickly or too slowly, you will have a natural speed at which you get more turn than at other speeds. You need to find this. Try bowling slower, and speeding it up, until you find a speed at which you get good turn (and you may well naturally get drift or dip!), and then continue with that. I find it difficult to believe that a person can "bowl too slowly", if that is their natural speed (Pragyan Ohja for example), only if their natural speed is quicker like Monty's is.
Also got plenty of links for you to read to help you!
http://www.pitchvision.com/spin-bowling-flight-and-guile - The string drill that I mentioned
http://www.pitchvision.com/how-to-improve-your-bowling-control-without-becoming-robotichttp://www.pitchvision.com/the-number-one-rule-in-spin-bowling - Highlights the need to "spin" a ball as hard as possible.
http://www.pitchvision.com/master-the-pivot-to-give-the-ball-extra-spin-revolutions - Pivoting on your foot - IMPORTANT!
http://www.pitchvision.com/6-proven-bowling-practices-that-are-better-than-having-nets - Useful drills
http://www.pitchvision.com/4-fixes-to-stop-you-bowling-full - Things to help you stop bowling full tosses
http://www.pitchvision.com/how-finger-spin-can-push-rpm-into-the-red - Talks abut really spinning the ball.
A whole host of things that will help you here, if you ever have the time to flick through them -
http://www.pitchvision.com/category/spin-bowlingBen