Some tricks for MySQL I learned

innodb_buffer_pool_size should be given 50% to 80% of total RAM. So that more tables could be cached in RAM, reducing chances of using hard drive and slowing down queries.
"Write-heavy tables should not use index" is a lousy statement. One of the fixes a team discovered to enhance an upload process speed is putting two indexes in a column to be used in an UPDATE JOIN statement. It SLAHSED the time taken by 99%. It doesn't mean to put indexes in every column, in every table. Test and verify.
If you have SELECT ... WHERE a = AND b =, index (a, b) is better than index a and index b separate. INDEX JOIN is expensive.
MySQL slow query log should be turned on to monitor for any slow queries. As with caching, or saving precomputed results of commonly used queries.
EXPLAIN and ANALYZE are your best friend. EXPLAIN will tell you what the database plans to do, while ANALYZE will run the query AND explain what it did.