[marbles]

When calculating probabilities on the flop, particularly "what's my probability of improving at all," it's often easier to calculate the probability of the opposite, then taking 1 minus the result.
Example:
You have 99. Flop is 49A. At this point, let's take A9 and AA out of the equation, and assume that any A, 9, or 4 will be a good thing. Since there are 7 such cards (3 A's, 3 4's, and one 9), the probability of success (call it 'p') is 7/47 (.149). Equally important, the probability of failure is 1-p, or .851.
Now, let's say the turn doesn't help you, and the board now reads 49A[X], [X] being any non-A, 9, or 4. Now your probability of success on the river (call it 'q') is 10/46 (can you see where the 10 and 46 come from?), or .217. This time, the probability of failure, 1-q, is .783.

Now, if you're on the flop, and you want to calculate the probability of improving at all, you could calculate:
The probability of improving on the turn and not the river, the probability of improving on the river and not the turn, and the probability of improving on both (adjusting the numbers for the different-looking river scenario).
Sum them all up, and you have tediously gotten your answer.

Simpler, though, you can calculate the probability of improving on NEITHER:
(1-p)*(1-q)=.851*.783=.666. Subract this number from 1, and you find that, as of the flop, you have a 33.3% chance of improvement.

Clear as mud?