I'm far from being very experienced at it, but since last year my training has been involving a huge share of so-called "internal exercises"; so here are the tips I've found to help when it comes to meditation:

1- After getting in the desired position, I'd say the first thing to do is to sort the breathing out. Keep the tip of you tongue connected to your palate (without really "pressing" it there either, since what you're trying to do is relax and any excessive tension goes against that), and breathe as peacefully as you can, trying to slow your breathing down gradually by making each breath (in and out) last longer. Don't slow it down to the point where you struggle to keep it so slow either, for that would mean unwanted tension in your chest (among other areas). When your breathing is both slow AND effortless, it's time to take care of step two.

2- Now to relax your body. As JKDWarrior pointed, the most simple way to do this is to run through your entire body like through a checklist. Concentrate on your feet (yeah, I personally run from toe to head, but JKDWarior's method of running from head to toe is just as good), and relax them as fully as you can. "Feet relaxed: check". Then proceed to focus on your ankles and relax them until you can say (mentally ) "ankles relaxed: check". And so on up to your chest, then shoulders, arms, wrists, fingers, neck, face. Once your entire body feels relaxed, a good way to keep it that way is to try and make your body to feel "heavy". Imagine that you're sitting on the sand at the bottom of the sea, and that you're trying to get your body to sink itself into the soft sand out of mere weight. That way, your body should instinctively avoid any motion or tension that would come in opposition to just simple gravity - and pretty much any muscular tension belong to that category (that's why lifting a person who's lost consciousness is, paradoxically, often harder than lifting an unwilling person).

3- OK, now for the meditation itself. Apparently, like me, you have trouble shutting off the flow of your thoughts. Just don't bother yourself about it, otherwise it will only make things worse. Instead, try to be more like "yeah, that's right; I still have a lot of rubbish going through my mind. So what?" . If you're at least comfortable about the restlessness of your mind, it has more chances to go away. You should even experience, now and then, some short (or not so short) 'blanks' when it DOES go away, and hopefully those blessed moments should become more and more consistent with time and experience. Now one of the most common weapons against those parasite idle thoughts is to concentrate on one given thing. Your breathing is an eligible candidate. If you're relaxed enough, you should even be able to perceive your own heartbeat very clearly, and that's another potential place for your mind to be (you can even try to influence it, and eventually become able to slow it down a bit). Some sentences can do that job too (For instance, in the Middle-Ages, would-be knights would spend the night kneeling and repeating some pater noster's or other prayers till dawn before being knighted). Just find what works for you.

4- Troubleshoot!
Sometimes, if you can't 'find it', the reason might be that some unwanted tension has been building up again somewhere. That's why I would advise to run through your 'checklist' again now and then, just to make sure some muscle hasn't re-tensed itself while you were not watching. Muscles of the face are among the usual suspects: it's VERY easy to unconsciously get into a frown when meditating, especially if things don't run as we'd like them to. If you meditate in a standing position (a common exercise in several styles of gongfu), legs will naturally have that tendency as well.
Another obvious cause that may affect your ability to meditate is your environment. See if there's anything wrong there. If you have the opportunity, there's no beating meditation in a natural environment (a garden is fine, wilderness is even better). However it has its limits: you should be comfortable, which will obviously be difficult in case of freezing cold, for instance. Home is usually the choice that comes just next, since it's usually the place where you'll feel safest and most relaxed.

5- An extra piece of advice I received from my instructor Mr.Zhou Jingxuan when I was learning gongfu in Tianjin. I'm rather clueless about the reason for this, but I just pass it down. According to him, you should always be facing east when doing any kind of meditation/neigong exercise. South is acceptable too, but north and west should be avoided. Don't ask.

6- Last but not least: persevere .

OK, that's about all I can think of now. I'll keep you posted if I think of more meditation tips.

I hope it helps!