First - get a front-clip harness. I use harnesses that are this style:
But instead of clipping the leash to the D-ring on the back (like in the picture), you clip it to the O-ring on the dog's chest. Voila, a front clip harness that you can get for under $10 at Wal Mart. I also think they fit more comfortably than, say, the Easy Walk harness.
Anyway, the front clip harness will serve as a kind of crutch, so that if she pulls you can stop walking or turn around; if she pulls on the leash, she won't be able to pull hard and it will help keep your hands from hurting. You'll have more endurance for a longer training session if you need to.
I'm sure you've read all about the "be a tree" and/or "choose to heel" methods of leash training we've written about a hundred times on this forum (if not, feel free to look them up), so I'm going to skip over the basics.
But one trick I use a lot with dogs that like to sniff, is to use sniffing as a reward for walking nicely. So you're walking along and she's walking nicely for a few steps - head up, not pulling, heeling right with you - and you're clicking/treating the good position. Have her walk a few feet nicely, and then give her a cue that means she can go and sniff; "go sniff" works well enough.
Use the sniffing as the reward for walking nicely. Let her sniff in a relatively small area, but as long as she wants to. When she's done sniffing and looks to you for something else to do, then cue her to walk with you and walk on.
Start cueing her to go sniff after just a couple of good steps, but the better she gets the longer you can go between sniffing sessions. Make sure that you cue it every time, so that she understands that the reward is coming from you.
A lot of terriers like to chase small animals, too, so if she does, you can also use that as a reward for good walking. She walks nicely and ignores a pigeon, the next pigeon you come to cue her, "go get him!" and let her chase the pigeon.
In my somewhat limited experience with terriers, I've found that this method is a great way to teach dogs that they CAN go and do those things that are so hard wired for them to do, but the
opportunity to do it comes from ME. It takes a lot more creativity than just giving the dog a treat or a toy, but with the right dogs in the right situations, it works BEAUTIFULLY.