I use Slime and I have never had a flat.
If I start getting flats I think I'll add some tire liners like Mr. Tuffy - but you really have to sand the edges or the Mr. Tuffy edge will puncture the tire after a time. I am not sure of the actual reason maybe it is a puncture or rubber stretching over the seam with every inflation. The end result is the same, you get a flat. Sanding is a pain. I've had good luck putting scotch tape over the seam where Mr. Tuffy meets itself. One thing I don't understand is why Mr Tuffy is so darn expensive for a 3 inch tire. I would have thought that by now somebody would have come up with some generic polymer tape as a cheap knockoff solution.
I'll stay away from the foam stuff - seems like a lot of hype to me because the foam flattens.
If you use an expensive tire pressure gauge, I suggest you don't use it because the Slime or Flat Out will ruin the gauge. I just let the air out of the tires and pump it up since I have a gauge in my tire pump.