I almost never have to adjust the thumbscrews on my bed, it seems to stay flat fairly well once adjusted, but I had to constantly reset the Z-stop screw because it vibrated loose all the time, when led to my invention of: http://www.thingiverse.com/thing:506638 which lets me use some blue locktite to keep the screw in one place.

Nice, I found that a little blue loctite on the bolt into the plastic works just fine.  Still adjustable with my fingers yet doesn't move.

Seems like ever few days I have to relevel my bed.
Why?
Are the thubmscrews getting loose?
Is the z-stop thumbwheel  getting loose?
How to prevent these?
Or maybe something else?

If you are having to re-level your bed there is a good chance the rivets or some other part of the platform that is loose.
I haven't leveled my bed in over 4 months.

I would recommend adjusting the 3 leveling screws as tight as you can get them without having the bed hit the platform.

If you are referring to adjusting the z-height adjustment then look for something that is loose in that area.

The bed (and as such the thumbscrews) will undergo vibration just by the nature of the moving parts on the printer, this will cause the screws to rotate little bits at a time and loosen off.  What may be worthwhile is replacing the thumbscrews with locking nuts (the ones with the non-metallic insert) this should help limit the amount to which the screws loosen off due to vibration.  (although admittedly it will be more annoying to level as you will need a wrench)

Tighten the leveling screws as far as they will go while keeping the bed level. I made a modified z stop for the SD4 so you can use a longer screw if needed. If you print it at 100% infill, it is sturdy enough that the force of the bed will not bend it and mess up your first layer; I have seen this happen with my stock z stop unfortunately. They tried to counteract this with a spring but it doesn't work.

It also helps to let the bed sit for a few minutes at temp before printing so everything can undergo thermal expansion and "re-align".

If you are over extruding, your bed gets knocked around a lot and this can contribute to falling out of level.

Overall your bed should not fall significantly out of level, I only have to balance mine once a month or so now.

Lots of info, hope it helps. The bed leveling systems is one of the reasons to not print directly on the kapton. You should see what scraping a print off can do to your perfectly level bed even if you're careful...