I am just going to repeat most of what's been already told.
1. Make sure that you have not spooled your reel till the lip
2. If you are using power pro, then 20 lb line on a slammer 560 is pretty light. Go heavy ....
3. Use a nice spro swivel to connect your braid to your leader
4. Thumb the spool at the end of the cast and manually close the bail. This eliminates a lot of slack
5. If you are using top water lure lures and such that does not have a tight resistance in the water, you can pinch the line with one hand while reeling in with the other. Be careful not to hurt yourself when a fish strikes and runs. Wear a glove if you are comfortable with that.
6. Last thing to look into is the spool taper:
If the line is wound in such a way that it is tapered toward the top of the spool, then that could the issue. But I doubt that this is the case with a Penn Slammer. If this is the case, then you can use some washers under the spool and have more line at the tip of the spool and slightly tapered toward the bottom. This will eliminate wind knots to a great extent. I personally prefer an even line lay though, as it looks neat, casts far and holds more line.