My pickpocket skill is low. Its still 28 while my character is level 44. I need a good place to raze my pickpocket skill without using invisibility. Any tips?

Just save right before you attempt to pickpocket someone and always steal jewelery it raises the skill considerably.

And of course pick the theif stone to raise it even more so while doing what pepe silvia said.

The Jarl's and their assistants usually carry valuable items/jewelry which can raise you pick-pocketing quite significantly.

Pick items with high value/weight ratio. Jewelery, gems, that sort of things. The more valuable item, the more XP it provides.

Yeah, pickpocketing is like the easiest skill in the game to train.

You want to set fire to your Pickpocketing skill? Why?

Wait until night time, and then break into every house in town and pickpocket all the sleeping people.

Silda the Unseen in Windhelm can also train you. I believe she is an expert trainer.

Pickpocket steel arrows from city guards. It's really easy and will quickly raise your thief skill (especially with the Thief Stone) until you can get a reasonable chance of success with more valuable items like jewelry.

Go in Riften, the bounty is smaller. There is also the thieves guild. If they catch you, they won't attack.

Every town and city is a prime target. Unsuspecting citizens walking around with jewels, rings, and gold sitting in their pockets.

Hit the Bards College in Solitude. There are a bunch of targets and they have pretty valuable stuff. You can do quite well for one building.