Check teeth, saddle fit and back if they haven't been already. If no problems found here then all you can do is sit deep and kick the horse on (sounds easier then done at times but hang in there). This should after time remove your horses bucking habit. Good luck
Keep them moving forwards - easier said than done I know but it worked for my girl. Still get the odd one for old times sake but can pretty much predict when its going to happen now - usually if she hasn't had a canter in a while and I don't keep my leg on!
In regards to riding dont let the begger get its head down ride forwards with positive aids.
I dont find shouting or whipping does them much good i tend to sit ride forward and ignore it.Even though using the whip or shouting at them is negative attention its attention all the same and i refuse to give it.
1) Get all the checks done, it could be ill fitting tack, bad teeth/bak causing the bucking.
2) When does your horse buck? Is it only in the winter months or all year round that your horse bucks? At this time of year because of the cold weather your horse could be cold backed and your not warming up enough for it. I do agree with Bugaboo though. Ride foreward, almost ignore that the buck even happened and just keep riding forewards. Some horses will use bucking as an evasion so if you keep riding the horse forewards they learn they can't get away with it.
Izzi
I agree with all said re checks etc. I had one whose bucking was resolved by using a sheepskin girth sleeve. She was a bit uptight so took a deep breath and this meant that her girth tightened and she hated that and bucked. it took us ages to work out why but an elasticated girth and a nice thick sleeve sorted it out and she was much happier. Always worth a try.