You don't. 300 is the cap for ALL SKILLS (I assume you mean professions).
You can boost them over 300, but these are not permanent. Let's see what I can think of:
Fishing: Boosts up to 427 (422 Alliance) with Nat Pagle's Rod, the Aquadynamic Fishing Lure, and the Fishing Enchant.
Skinning: Boosts to 315 with Finkle's Skinner and the Skinning Enchant.
Mining: Boosts to 305 (?) with Mining Enchant.
Engineering can be 315 with Gnomish racial ability. Herbalism can be 315 with Tauren racial ability. I think herbalism also has an enchant of +2 or +5.
That's all that comes to mind. Each of these are temporary, and not your real skill (well, racial isn't temporary, but the rest are :-P).
If you're confused by allakhazam recipes that seem to say "315 skill", they actually turn green at that level... or would, if you could go that high. Recipes only go up to level 300.