Write a lot and read a lot. You can't really be the best writer possible without reading, and believe it or not, reading helps to subconsciously ingrain spelling and grammar into your head. You wouldn't believe how important that is, simply because a publisher won't usually even look at a manuscript unless it has decent spelling and grammar.
And of course you have to keep your writing skills sharp. I recommend writing daily, even if it's just for 30 minutes some days.