Sorry for the really late reply, I don't go on this site all that often.
Generally you want to place a pink somewhere where it is unlikely to be seen, but there are exceptions. And remember, it's better for an enemy to see your pink ward and kill it while you have time to retreat to safety than it is for them to simply gank and kill you.
Anyway, the best places to put pinks depends on your role but generally it's best to place them in the enemy jungle to keep track of where they are. Putting one behind their red buff in the bush is one very good place and another is in the tri-bush near their blue.