1) Analyze your indexes on all tables. Start with the high volume insert/read tables.
2) Look at your slow query log. Pick the slowest three queries. Optimize them. Setup a recurring meeting with your team to do this every week.
3) Start looking at what data can be flattened and is less important and could be stuck in a key value datastore.
4) Consider loading a replica slave server. Off load some of high volume read queries to the replica. Also serves as redundancy.
5) Consider where it may make sense to move tables to InnoDB. Table-level locking can be extremely time consuming when traffic is high. Row-level locking can help here.
6) Figure out what your hardware upgrade path looks like. You will have to do this eventually. Identify when you might hit a hardware ceiling with your hosting provider and get a plan in place. Coordinate with your marketing/sales team and upgrade before you hit a traffic peak, not during.