To keep the dog from sitting, spend more time rewarding the dog for standing than you do for sitting.
If your dog is sitting each time it comes up to you to get a reward, you need to stop rewarding it when it is sitting and start rewarding it when it is standing.
Teach a verbal and a physical cue for standing. If a dog sits in front of me when I want it to stand, I will lean in towards it, say "get back" and if it does not stand then I might touch (GENTLY!!) a rear foot with the tip of my toe. As soon as the pop up into a stand, they get smiles and treats galore. Sitting makes the smiles and the treats go away when I am show training, and then we start again.
Gaiting is much the same. Dogs must learn to stay on the left, keep a steady pace, and if possible, not pull on the lead. It's all practice.
If you intend to show a dog, a handling class would be some money really well spent!