I've noticed that people who have a hard time in intro-to-physics classes usually fail in a fairly predictable way: they see a problem, don't know how to solve it, and stop. But there's a trick to solving physics problems when you're not sure how. The general method is:

1. Make a diagram or something, to depict what you know about the problem and make it easier to figure out stuff you don't know.

2. Look at the problem until you can derive something you don't already know. Even if you have no idea how it will help (or if it will help) do the calculations, and write them down.

3. Repeat step 2 until you see how to get to the answer.

The problem is when someone, not seeing a way all the way to the solution, stops trying instead of looking to make incremental progress.

I'm sure this applied to more than introductory physics problems. It's very similar to Terry Tao's advice on how to do math:

I don't have any magical ability. I look at a problem, and it looks something like one I've done before; I think maybe the idea that worked before will work here. Nothing's working out; then you think of a small trick that makes it a little better but still is not quite right. I play with the problem, and after a while, I figure out what's going on.

Most people, faced with a math problem, will try to solve the problem directly. Even if they get it, they might not understand exactly what they did. Before I work out any details, I work on the strategy. Once you have a strategy, a very complicated problem can split up into a lot of mini-problems. I've never really been satisfied with just solving the problem. I want to see what happens if I make some changes; will it still work? If you experiment enough, you get a deeper understanding. After a while, when something similar comes along, you get an idea of what works and what doesn't work.

It's not about being smart or even fast. It's like climbing a cliff: If you're very strong and quick and have a lot of rope, it helps, but you need to devise a good route to get up there. Doing calculations quickly and knowing a lot of facts are like a rock climber with strength, quickness and good tools. You still need a plan — that's the hard part — and you have to see the bigger picture.