One thing I like to do when I complete a deal by notifying the forum member that I've either shipped the item, or received it if I'm the buyer, is mention that I made sure to have left feedback for them.
I feel this subtly helps remind the other person to leave feedback for you as well. And I think it lets the person know I was pleased with the transaction.
Most times I think it is just slips a guys mind as there is usually a several day delay involved in the deal waiting on shipping.