First, I never have the guard on my weed eater except when using the saw blade. Second, I use the following technique for tall grass:
I start at the edge of the area I'm working, i.e., I'm standing in the tall grass and reaching OVER the grass so the back side of the trimmer head is doing the cutting. Then, I start working it backwards, towards me, and I sweep side to side as I walk backwards. If there's any short stuff left after the first pass, it is then easy to clean that up without anything wrapping around the head.
This throws the grass out the front of the trimmer head, away from the trimmer, so it doesn't wrap around the head and gets rid of most of the tall stuff. I work in sections like this, whatever I can reach from one spot, may be 6 feet square. Then, mover over and start the next section.
Once you are used to it, you can start from outside the tall grass, again, reaching over from the top and lowering the head down slowly to create a "hole" where you can start using this technique.
Removing the guard also makes it much easier to trim around things like trees and fence posts because you can reach around the post and trim with the back side of the head. You can also reach OVER things easier, like when trimming the opposite side of a ditch.