I don't like having my belly button touched. There, I said it. I have no belly-button-based trauma, I just find the feeling unpleasant and I react very poorly to anyone going near it. Like, screaming and desperately flailing poorly. Most people I know have a similar response to one or more particular areas of their bodies, and accept that that's just the way it is. And yes, polling acquaintances was awkward, but these are the things I do for you, dear reader.

So, if we're willing to accept that other humans have spots they'd prefer we avoided, why is it that so many people can't seem to grasp that dogs feel the same way? The exact spot (and severity of reaction) will vary dog to dog based on individual preferences and past experiences, but I've yet to meet a single critter who was just naturally content to be touched everywhere.

Why is this important enough to write about? Well, think of all the times in her life your dog will need to be touched in particular spots, often by strangers. Every vet visit, every trip to the groomer, every small, clumsy, grabby child who wants to say hi to "the goggy" throws up opportunities for contact with her No Go Zone.

Now, if she's a particularly tolerant sort, she may initially only turn away, freeze or lick her lips - signs we often miss (I can't tell you how often I've seen dogs asking very politely for the touching to stop, only to have the owner say "No, she LOVES this!" when I point it out). If your dog is NOT the tolerant sort (or is perhaps in discomfort of which you are not aware) she may forgo the courtesy and head straight for the warning shouts of growling and snapping.

Obviously this is less than ideal; even if she doesn't actually hurt anyone she will likely have to wear a muzzle for all appointments (which only makes the experience worse for her), and have her activities curtailed if she can't be trusted around children - this is of course on top of the fact that she is so distressed she feels the need to defend herself!

"Ok" I hear you saying "Fair point and all that, but what can I actually do?" The answer is simple: it's your responsibility to teach your pup that being touched all over is not only ok, it's a good thing. We do this through pairing a mild stimulus with a reward to build up positive associations.

Begin with a relaxed critter, and slowly and gently run your hands over her, taking note of every time she freezes up, looks away from you, licks her lips or tries to move away (some common bug bears are: top of the head, ears, teeth, paws, nails, tail and butt-area).
If she does any of those things, immediately stop touching her there, and give her a little break by either backing off or moving to tickle a spot she particularly enjoys. For example, my furball can be sensitive about her ears, so I would stop as soon as she twitched away from me and start scratching the base of her tail instead (her personal idea of heaven).
Once you have a good idea of her No Touchies, you can start working on desensitising her to contact with them. You'll need some high value treats for this, so be prepared!
If it's an area to which she does not react really strongly, gently touch the spot for a second whilst praising her, release and immediately give her a treat
If it's something she reacts quite strongly to, feed her a treat and praise her as you gently touch it for a second. Stop before she finishes the treat.
If she has a really strong reaction (typically the result of a history of bad associations or discomfort because of illness or injury), either feed her a treat and praise whilst you move your hand in the general direction, or move your hand, praise and then treat (depending on the severity of the reaction). Do not make physical contact.
As she gets more and more comfortable with these steps you can very gradually increase the length of time/intensity or the exercise.
It's important that you do not push past her stress threshold whilst doing any of these exercises - she should be comfortable enough to be voluntarily staying next to you and taking treats. If she can not, you have pushed her too far - stop immediately.
Once she is comfortable with you (someone she presumably already trusts), start having strangers perfom the same exercises in the same order - remember that she will not have generalised her acceptance of the touch yet, so they need to go just as slowly as you did. You should also practice them in a variety of environments for the same reason. The more people and environments she gets used to, the less chance there is that a new person touching her in a new context will worry her.
If possible, have your vet and groomer work on these exercises too. If they are unwilling or unable, ask if you can practice the exercises yourself in their premises.

As an example, my rescue dog came to me with a bad ear infection. She was therefore already sensitive to having her ears touched (because they hurt), she didn't know me at all, and I was not able to build up trust or desensitise her before having to meticulously apply and rub in the ear drops 4 times a day: practically the perfect storm of creating a touch phobia. Fortunately she's a tolerant soul. Unfortunately, after that experience she was understandably emphatic that her ears were Not To Be Messed With. It's taken 4 years of constant and gentle desensitisation for her to be comfortable with me not only touching her head but fondling her ears and manipulating them to look or clean inside. If she's having a bad day and/or they are bothering her, her tolerance decreases considerably. She also loathes having anyone bar my partner or myself go near them, so vet visits can be stressful and require a constant stream of praise and rewards just to help her maintain emotional equilibrium.

In contrast, she had no issues with her paws beyond the usual mild "Why are you touching them? No please." response and because we were able to take our time working on them we (and any one else) can now touch them, trim her nails and inter-pad fur, and apply snow booties with no drama. As an added bonus, when she stepped in tar (during a hot New York summer) she had a sufficient backlog of good experiences with paw handling that us soaking and peeling the wretched stuff out, whilst not her FAVOURITE activity, did not make her reactive after the fact.

I really cannot overstate the importance of doing this for your beasties - it takes so little effort (5 or so minutes a day) and so dramatically impacts their quality of life, so why would you not?