I have about a 70% success rate with aluminum, success being it looks fairly straight and sounds right, and isn't twisted; it may have a little kink or dent, which is annoying every time I see it, but doesn't affect play. Failure means I broke the damn thing off, which happened the first two times I tried — I'm surprised I had the guts to try again, but I analyzed my technique and corrected it. Success.
If "bent" means a sharp angle at a single point (as opposed to a gentler curve), and you apply force at that point to bend it back, it's going to break, because it's already partly fractured. So work around that point as much as you can — I mean bend other parts of the cantilever, and bend them just a little. If the total bend is, say, 10° off center, a series of small bends that add up to 10° is the safest approach.
To apply force only to a small area, you need something to bend against (call it a "fulcrum") — it not only focuses the force where you want it, it also prevents the compliant suspension from absorbing the force. Jfrace mentions a sewing needle, it's what I use too. I had a van den Hul bent downwards like you describe. Instead of the normal 15° angle, it was like 25°. That's bad — and working on a VDH is more nerve-racking than a $20 AT. Luckily it was aluminum — also, I was able to "nude" the cartridge, exposing more cantilever to work on. Putting the needle under the cantilever to hold it firm, and applying slight force at a series of points (moving the needle each time), I got it back close to 15° (judging by eye, when it's so tiny, who can tell?). Sounded almost like it did before, and adjusting VTA at the tonearm base fixed the small error that was left.
Tweezers can also focus the force on a single point — But I try to avoid "serious" tools because they're all force-multipliers, making a tiny slip catastrophic.
Also — know when to stop. If it's not perfect, but it's close, STOP — don't try "one more little" thing. You can correct the error that's left by how you mount it in the headshell. And "how it looks" doesn't matter when it comes to playing it — it may have little kinks and dents, but if the stylus is in the right place it will play right. Then just forget about it, you can't see the cantilever anyway unless you lie on your back on the floor looking up at it.
Sorry it took me so many words — TLDR — must hone my writing skills...