I'll cut to the chase; there is a simple change to your bowling action that is a big hitter in fault correction.
Straighten your run up.
You see, when it comes to technique, so much that goes wrong can be traced back to an earlier point. That's why batting coaches focus on the grip and stance first, and it's why your bowling coach should look at your run up before he starts with the "business end" of the action.
Of course, a straighter run up will not fix everything, and there are exceptions to the rule. That said, there is plenty that can be done without ever worrying about 6 months of corrective drills and rebuilding your action.
So, spinner or seamer, Here are some of the things a straighter run up can correct:
"Balance at the crease" is a coaches way of saying "helps you bowl straight".
When you run straight you tend to move through your action straight. If you approach at an angle, your weight is desperate to continue on that angle (say, towards leg slip).
To correct this, your body naturally adjusts and you end up "falling away"; the term used when your head leans to the side to allow your arm to come through straight.
When this happens it is much harder to bowl straight.
By running straighter, you make it easy to bowl straight. Your head can go over your front leg and you can get your wrist behind the ball. Your action becomes repeatable and so does your line.
I'll let you into a little coaching secret; a lot of pace - and turn for spinners - is generated by your hips. The more powerfully you can drive your hip through as you bowl, the more energy you can put into kph or rpm.
How does the run up help with this?
Many bowler's have an issue of being too "closed off". This is where the front leg crosses over past the back leg. This stops you from driving your hip through because your front leg has, literally, closed off the path for your hip. The result; you bowl with less pace.
And it's much more common in bowlers with off centre run ups because your weight is moving towards leg slip. Straightening up your run straightens your momentum. Your feet can get into the right place for more speed or rip.
Exceptions to the rule
Of course, so far we have only spoken in generalisations. There are many specific exceptions. There are bowlers with curved run ups who bowl with pace and accuracy who sit alongside straight run up bowlers with neither.
So the first question to ask is this: do I have an issue with pace or accuracy?
If you bowl at a good lick and can hit your lines well with a curved run up, you should probably keep it. It actually helps with swing and spin bowling sometimes.
If you feel you need to make a change, a straighter run up is very often the answer you are seeking.
image credits: brianac37. Gordon Anderson