Some good info in this thread about topping, fimming, supercropping and side lighting. All these methods will promote extra branching one way or another.
Topping and fimming are pretty stressful to the plant though, so should really be done in veg, with enough time to recover before flower. In my opinion, the earlier you top/fim, the better the result, with regards to bushiness.
Supercropping also needs some recovery time, but less so, and it has the advantage of not having to chop bits off your plant. I regularly supercrop my mother plants, and as a result i have like 10+ main growth tips at a time to choose from.
One technique that is missing from this thread is LST (ie:- tying the bitch down). This can be done temporarily, by tying the main growth tip below the others, giving them a chance to catch up, or on a permanent basis, with multiple ties to keep the plants short, with no one tip being more dominant than the others. LST gives some REALLY bushy plants when done properly (sort of like ScroG, but without the net). It is also less stressful to the plant than cutting or breaking branches.