Keep it simple, although less casual than usual. Pursue more romantic activities while you are together.
That does not mean, however, old partners, intimate gossip, school, or anything else that you usually talk about. If your friendship up until that point had been extremely fun and witty and open, then expect that to continue. This will make you appreciate each other even more while reducing the risk of becoming annoying to each other. A lot of guys we meet are wondering about dating a best friend. Instead, try to call them pet names that convey that you think they are special or attractive. Don't be in a hurry to change their status with them until you are more comfortable with the relationship.