I've barely practiced for the ACT, but I've taken the SAT twice and am hoping that experience will be useful. I'm worried though. Will the experience of having taken the SAT help me at all with the ACT?
It depends if you've taken the March 2016 updated SAT or the old SAT. The old SAT did cover some of the same materials as the ACT, but the pacing on the two tests is/was very different. The current SAT, on the other hand, is much more similar to the ACT, which means that prep that you did for the SAT could very well carry over into being helpful for the ACT.
Taking the SAT will also be helpful in the sense that every time you take a standardized test, you get better at taking a standardized test - you get practice filling out your answer sheets, listening to and quickly comprehending the proctor's instructions, focusing on the task at hand in concentrated bursts and using your breaks well, and knowing when your testing situation is out of the ordinary and needs to be reported.
In general, though, just as getting a high score on the SAT doesn't guarantee you a high score on the ACT, taking the SAT twice doesn't necessarily prepare you to take the ACT. As we've said on the PrepScholar blog, the best way to prepare for the SAT or ACT is to study using actual test materials. The SAT and ACT do have some differences in the ways they'll ask you about material, and while the ACT tends to be more straightforward, if you're used to one sort of questions you might still get tripped up (even if it is supposed to be "easier").