Pick Slippage--

Here are a few tricks to help with the pick slippage.
1-Drill/Punch Holes (already mentioned)
2-If you smoke, rub ash on your fingers (dries the sweat)
3-Take an emory board and rough up the edges that your thumb grips.
4-Carpet tape

Strumming-- Try these as warm ups for 1 month.

Pick a chord, any chord.

Play all down strokes

1 measure each of quarter notes, eighth notes, Triplets, 16th notes. Play slowly and evenly and focus on hitting/hearing all strings clearly.

Play down and up strokes.

Same as Above.

Play down and up strokes.

Same as above but this time palm mute/ or "chunk" the chord every other beat.

Example.

D U(X) D U(X)

Then Reverse the last 2 exersises. Start with an Up stroke first and alternate between the Up stroke then Down stroke.

Pretty basic exercise but if you are persistant in your practicing and really focus on playing the strings fully and not cut corners this should help.

My 2 cents.

Good Luck and Rock on.