Use a pot a little bigger, not huge though. Vancefish has a good idea, but a couple things to keep in mind:

1- the pot it is in now looks pretty big, not sure how deep it is, but the roots probably haven't filled it in much yet. This might make transplant difficult, and cause stress if the rootball breaks apart.

2- you don't want to keep the plant in the same soil/container too long (more than 6-8 weeks), otherwise you will have problems. I would do a final transplant a couple weeks into flowering (maybe 1-2 months from now...) using a 5 gal bucket to finish your grow. Between then and now, transplant into a 3 gal or something in between what you have now and the 5 gal you finish in.

Just some different opinions, transplanting like Vancefish said should work fine also, and will help to train and bush out the plant.

For training, I use nylon string. Doesn't seem to do too much damage. Remember not to tie it tightly to the plant, as the stem will get thicker. I cut a piece of nylon string long enough to reach from the anchor point on the bucket to the plant, and then back to the anchor point. That way as the stem grows, it will not get strangled by the string.