Fast levelling a gathering skill like herbalism requires you to be higher level than the mobs that 'guard' the herb spawns or to have an escort along (ie a pet or another player to take aggro while you gather). Once you've got the mob-clearing aspect taken care off the other simple tricks are:
- gather only during off-hours, as during primetime there is too much competition
- get a mount, it makes a big difference
- use Gatherer mod to track where the spawns are and hit them regularly