I carry this pistol a lot. During the day what with bumping it and other brushes against stuff, the safety becomes disengaged. There is not much tension to hold the safety in position. Is there someway I can tighten the manual safety? I'm always checking to make sure the safety is on.

Yes there is, if you comfortable with the disassembly of the sear block. There are notchs on the safety lever inside the pistol that can be deepened using a small round file, which will allow the safety detent block to engage more positively.
In order to do the modification, the sear block / safety assy must be removed from the pistol.
If you are not comfortable removing the sear block assy yourself, then any competent gunsmith could do the mod.
I have multiple M&P with safeties, and I have mod'd all of them to get a more positive feel.

Thank you for the info. I will do research on how to do this myself.

When you remove the safety lever to deepen the notches, be aware that there is a spring loaded piece that engages the notch. It will fly out if you're not careful, never to be seen again. You might want to think about disassembling the sear block and safety assembly in a plastic freezer bag or somesuch to,catch it if it goes flying.