Best practice to record large amount of hits into MySQL database

500k daily it's just 5-7 queries per second. If each request will be served for 0.2 sec, then you will have almost 0 simultaneous queries, so there is nothing to worry about.
Even if you will have 5 times more users - all should work fine.
You can just use INSERT DELAYED and tune your mysql.
About tuning: http://www.day32.com/MySQL/ - there is very useful script (will change nothing, just show you the tips how to optimize settings).

You can use memcache or APC to write log there first, but with using INSERT DELAYED MySQL will do almost same work, and will do it better :)

Do not use files for this. DB will serve locks much better, than PHP. It's not so trivial to write effective mutexes, so let DB (or memcache, APC) do this work.

A frequently used solution:

You could implement an counter in memcached which you increment on an visit, and push an update to the database for every 100 (or 1000) hits.

We do this by storing locally on each server to CSV, then having a minutely cron job to push the entries into the database. This is to avoid needing a highly available MySQL database more than anything - the database should be able to cope with that volume of inserts without a problem.