Keep them motivated, working on interesting things, help them continue their growth even if that growth means they're leaving the company. And the harder part in some ways... tell them your own aspiratations for growth and the potential for you to move up (and/or on) yourself.
Honestly, pay them more. You can also look at getting them MBA interns for the summer, etc. Also, depending on how big your marketing team is, you can give them expanded scope outside of PMM, if that interests them. e.g. have them manage a couple copywriters who are contractors.
Improve their skills. Prepare them to replace you (so you can get promoted). Or prepare them for their next opportunity. Help them build their resume. Not only is it the right thing to do, it builds loyalty.