I agree with slugster's "if an attacker grabs your breasts then s/he has at least one hand extended which you can use against him"*, and almost all of his general advice, but think "Using your hands or forearms you can trap the hands against your chest." should be at most a momentary measure while you get in a good strike. If the attacker has grabbed you and you trap their hand(s), there's generally nothing to stop them applying even more pressure, so you don't want to be there for seconds or tens of seconds just holding/trapping their hands, getting hold of one of their fingers, or trying to twist free. The best you can hope for is that one of your arms can momentarily pin both of theirs (if they've grabbed you with both), or at least their dominant hand, leaving your other arm free to strike their face or throat, and your legs free to kick.
For example, if they'd put their hands on both your breasts, and you're right handed, you could bring your left hand up and over their arms so one of their hands is pinned by your left bicep and you have your left hand is on top of their left arm, then hold their arms there while you strike (repeatedly if necessary) over their arms with your right hand and/or kick through underneath the arms. Your left hand is thereby stopping them using their arms to intercept your strikes or hit you back, and also keeping their head at what's generally a good striking distance (if they're much taller, you may need to use your left hand to drag down closer to their elbow, so their arms bend and they come in closer, but it's harder to do effectively). Thrusting your palm to the chin or nose, or even slapping - using the palm halfway up the jawbone to twist the head sharply, are techniques worth considering. If you haven't trained serious and conditioned your fist, then your palm's less likely to get injured during striking. If they're really close, remember you have elbows, but they have very limited reach so they're harder to use than you might expect. Remember you can kick underneath all these tangled arms, and they're relatively unlikely to see it coming, though they may feel you shifting your weight to prepare to kick; just lifting your knee then snapping out a kick to their knee is faster, less "telegraphed" (obvious early warning signs for them) and has better balance and recovery time than trying some kind of football kick or going for the groin, though little in fighting goes according to a script and taking the best opportunities you get is important. Keep a clear head and don't get bogged down mentally in one way of defending or attacking - or even the fight at the expense of considering your environment and escape/call-for-help options.
A frontal grab's not dissimilar to a strangulation situation, where if you're weaker and try to grab their hands or get in under their fingers and make them let go, you may run out of time and air. Similarly, there are "nice" textbook techniques to twist this way or that and escape a grab or strangulation, but if they've pushed you up against anything, dragged you in close to them, put you on your back or hauled you up onto tip-toes, you may not have the leg contact into the ground to power a twist of the hips and torso to make "joint locking" techniques - those that twist a limb or joint and apply pressures in painful ways - work.
So, do as little trapping / joint locking as possible to be able to strike effectively; after striking you could do more, but if you're untrained in those techniques I wouldn't recommend it.
If you think you'll likely be followed and there's nowhere safe to run to, make more strikes count while you have the advantage of surprise and position, but run if and when you can, call for help, pick up some kind of weapon (but beware escalating the level of violence) or whatever seems the best use of the time window your strikes create.
Escaping a rear grab
If you're grabbed from behind, and their arms are under yours - turning and elbowing to the head is worth consideration. You can kick backwards at their knees, or down onto their feet. If they've grabbed around your arms - pressing them to your sides it's harder to escape from - the kicks are still good options, while a "textbook" escape is to drop your body weight while pushing both arms forward (as if you were doing a push-up with your elbows tucked in to your ribs); don't lift your elbows sideways like you're pretending to be a bird flapping its wings - that uses much weaker muscles and has a lot less chance of breaking the hold. Once the grib loosens, again look to run, or failing that strike. A problem with this escape is that it relies on suddenly dropping your body weight helping you slip out under their arms, but if they've already taken most of your body weight by bear hugging you to their chest and lifting you upwards, they're already supporting your weight and you can't drop. But you can kick backwards at their knees / shins, or possibly down onto their feet, and they can't hit you while they're lifting you up in a bear hug.
A final desperate measure that's got nearly as much potential to hurt you as your attacker is a reverse headbutt - if you catch their forehead you may injure yourself worse than them, but if it's your only chance and things look grim, it's worth keeping in mind.
Getting dragged to a car etc.
If they've grabbed you from behind, around/over your arms, then drag you backwards, the "push-up" like arm thrust I mentioned above as a textbook escape from a rear bear-hug should also help you loosen their arms. If they lift you while pinning your arms - whether you're facing them or away - there's little you can do except kick (or possibly bite or headbutt - both desperate measures). If they drag you by the hair, there are ways to grab their hands and joint-lock their arms but they're too elaborate to explain and hard to execute - you'd need proper training to have a hope of using them, so I can only suggest instead of struggling to get away, push in close and strike as best you can until they let go of your hair. Again, a hand used to hold you (even by the hair) is an arm they can't block or attack freely with, but keep in mind it's also a way to control your balance and movement.
Do remember that even with years of martial arts training it's not reliably "safe" or trivial to handle real-world fights - especially against stronger attackers - so don't over-estimate how useful tips like these are. Avoiding danger's always the best bet when possible.