There are two ways to go about it. To fix the immediate problem, don't stop walking. I'm not sure of your set-up, but I recommend a collar that can't slip off or pop open, possibly one that tightens when tension is applied to prevent this. You should also have an approximately 6' lead that is at least a couple of inches wide. The reason I recommend these is because they are the best tools for communicating with your dog. He can easily pull against you with a harness on, which were designed for tracking dogs. It'll burn your hands to try and hold him if he tries to pull against you and you have one of the thin retractable leashes.
My theory here is similar to what Ceasar Milan teaches. I feel that it's natural for a dog to follow it's leader and that the pack would never get anywhere in the wild if each pack member stopped to sniff what they wanted to sniff. Under that principle, I don't let my dog wander when I go on a walk. I want his shoulder at my knee, so that I'm directing where we're going and he can move out of my way if I make a turn into him.
If he starts to stop and sniff something I just keep walking. The slack is removed from the leash and he gets a little tug. He then trots to catch up. It's not going to be perfect at first, because nothing is. He'll try to stop and sniff a lot and he'll also try to run ahead. It's fine. Work on it a little every day. I wouldn't worry about tugging on the leash. It isn't going to cause a behavior problem. I think of it like tapping someone on the shoulder. I ask the dog to look at me or come on with my voice and if I'm ignored, I give a few little bump on the lead to get their attention. It's what it's there for, to be an extension of your arm when they don't listen to your voice. Eventually, if you always give a verbal cue first, you don't have to enforce with the lead, because they know you will and don't bother balking.
I won't worry about your dog not getting to smell, either. Watch any of the documentaries on tv and you'll see how amazing their noses are. They can walk beside you and still get to smell everything they wont to. They just won't be able to stop and investigate. It's similar to a little kid wanting to stop at every candy and toy store they see, so the parent tells them if they stop asking they'll take them to get ice cream.
Your other option is to teach your dog to heel. This is a whole other thing that I don't think is appropriate to type out here, but there are probably multiple sources to look up how best to do it. I don't know if there is an IM on this site, but I can give you advice on that this way if you need it. It's the same principle, though. You don't expect perfection at first, work in baby steps, make it easy, such as using a wall to block their avenues of movement, and reward often. Your verbal and physical cues come first and helpful physical corrections come second. Good luck.
