The best way that has worked for me is to send emails once or twice a week. I use their VP Ledgers as a guide to tell me who is working the business and who is not.
I believe in rewarding performance so reviewing the VP Ledger is the key. If I see high performance I will try to reward the person by re-assigning one or two good PSAs to them.
I like to advize the high performers with an e-mail that alerts them to the award I am providing.
