one of the big things is to understand what's going to happen with the cueball. I know that sounds like dumb obvious advice but so many players think they know what's going to happen, but they don't. They don't know what cue ball paths are realistic and what's not. A big problem I used to have is that I tried to bring the cue ball to positions that just aren't possible without crazy spin or force. You have to understand when you have too much or too little angle to move the cue ball the way you want, and adapt accordingly.
I dunno where you're at in skill level but here are a bunch of things that helped me so much when I finally got them. Some of them were like instant revelations, like I got them 1 day and it totally changed my game. Others I sorta always knew, but they snuck into my game more and more over several years.
1. Before all else, know where the cue ball will go with a firm center ball hit, which boils down to knowing the 90 degree rule. Understand that this rule only applies when the cue ball is moving fast enough that it's sliding (not rolling) across the cloth.
http://www.youtube.com/watch?v=1BDablltCQQ&feature=related
2. Know where it will go if the cue ball is rolling naturally (not sliding), which you can sorta estimate with what people call the 30 degree rule. Figuring this one takes some experience and basically hitting a million balls.
http://www.youtube.com/watch?v=yHhuSK1GdOI&feature=related
3. Understand that smacking left or right english on the cue ball doesn't affect how much it goes left or right immediately after it hits the object ball. Sidespin is made to control how the cueball reacts off the RAILS. Not how it reacts off other balls. There are subtle exceptions to this rule but basically it's true 95 percent of the time. If you're straight in and you whack the cue ball with left english, you aren't going to make it go left. Or you might but you'll miss the shot.
3b. How much you cut the object ball and how far above or below center affect the direction of the cue ball more than anything. If you're almost straight in and you want the cue ball to spin forward and to the left... you need to cut the object ball as much as possible (without missing it) while hitting the cueball just a little bit above center. If you hit way above center thinking 'more spin = better chance of moving the cb the way I want' you'll fail. If you thinking going nuts with left or high left english will help you move the cueball left you'll fail. The key is knowing that the more towards the center you hit on the CB, the more sideways it will go before the english (whether it's topspin or draw) catches.
4. when you're making rail cuts and you want to get the ball back towards you using low outside, or you need it to race around the table with running english, it's important to know that SIDESPIN matters a buttload more than follow or draw when moving the cue ball off a rail. This is important and it took me a while to get it so I'll say it again. It's sidespin that will get the most work done... not the follow or draw, especially for cuts near or greater than 45 degrees. That's not to say don't bother adding any follow or draw, but focus on the side. Unfortunately that's harder to aim than just follow or draw, but that's the price you have to pay to get the position you want.
Buddy Hall illustrates it nicely in one of the chalk-off videos. They've been deleted from youtube sadly, the video I had in mind was #10.
5. Try to make leaves where what you want to do doesn't require a lot of force, has lots of room for error, and sets up for easy stop shots or almost-stop shots. Avoid paths where you have to weave the cue ball through other balls, play the speed perfectly, pound the ball really hard, or barely feather it in.
Lastly, don't bullshit yourself on what's required. If the angle is too steep to 'hold' the ball for position, plan on moving the cueball back and forth across the table as necessary, don't baby the shot in and try to hold it anyway. If there's not enough angle to move the cue ball off the rail, consider rail first, or just settle for shooting off the rail. Don't pound the ball hoping for a miracle, you'll just rattle it. If you can't get from point A to point B no matter what spin you put on the ball, consider playing safe even if you have a perfectly easy sinkable ball in front of you.
I could go on and on but basically most position problems boil down to a problem with someone's understanding and planning, not their ability to stroke the cue ball and put spin on it. The more you understand, the less stroke ability you'll need.
There's a ton of good free video on youtube and the major pool sites, I recommend checking that out.