Here's what me and my friend do...
He wears his messenger bag and to not look suspicious he has giant headphones and a gameboy so he looks like the type that would have some weird bag.
So anyway, we go to Lowe's/Home Depot. Go to the paint aisle. Stare at it. And I take out my phone and act like I am on the phone with my dad or something and he needs a particular type of paint.
So I just act like i am listing off all the kinds of paint there to whoever is on the line. (is is just to cover up a reason why we are in the aisle so long and waiting for people to not look) My friend will stand there and i will kinda cover him and when no ones looking we just stuff some rusto in there.
Then we walk out.
Oh and our Lowe's, at least, doesn't ask if you're 18 if you buy paint, so... to look ultra less suspicious... just buy a can or two while you have 7 or 8 in your bag.