Other than a few general things, like making sure you have a fixed price before setting off, and that as a non-local you're unlikely to ever get a journey on the meter (which is used for locals in Bangalore at least), here's my experiences over a few trips.
Firstly, location matters. If you're stood outside of a nice hotel, then expect prices to be jacked up. Outside a very nice hotel, even it's just crazy! If you're staying somewhere like that, be aware that all the auto drivers will think you're immensely wealthy for being able to stay there, and they'll probably have inflated ideas about room costs too, so they'll charge a lot. Consider walking a few hundred meters away (but not to outside another hotel!) to get one.
Next up, trying to haggle with a few drivers who are all parked up together and are chatting will be a struggle. Especially outside of a nice hotel, they'll all agree with one-another on what a "reasonable" price is. In that situation, you'll probably have to walk away and flag down a passing auto, then you can play the two drivers off against each other to get a fair price. A cluster of autos outside somewhere popular will probably be different, and you can haggle between drivers there, but in cases where they're "staking out" a touristy spot, that won't work.
Direction matters. Both in terms of traffic, and in terms of availability of a return fare. Expect to pay more to go from a busy area out to your hotel, than you would from your hotel / office / etc into a busy shopping / eating area. Expect to pay more at rush hour, because of the waits, and expect to pay more if there's one way systems against you. For the latter, it can be worth a short walk against traffic to a junction to get an auto, to avoid having to ride the wrong way (and the cost!)
If you're getting local friends to haggle for you, make sure they let the driver know that you'll be the passenger. "How much for him to X" is fine. "How much to X" when you're not in sight can cause problems, with the driver then saying "that price was for your local friend, it's more for you".
Make sure you know the address of where you want to go, not just the name, along with the name or address of somewhere big nearby. Also, make sure you can pronounce these places like a local would (it can sometimes differ a lot). If you only know the name of the place, you'll probably be restricted to a small subset of drivers who normally work with tourists, at their associated markup. "Can you take me to Bar X" is tricky, while "Indiranagar, 100 foot road, at 12th main road, near the Sony World" will get you to the bars easier! If you can get close, most drivers will stop and ask for detailed directions, but you need to convince them to get close enough, which means a rough address at least.
Be aware of how low the price can go. If you've haggled hard to get somewhere for Rs. 100, and a different driver offers to take you back for Rs. 50 as an opening price, there's something dodgy going on. Probably they're planning to take you to visit to a tourist shop or two, so make sure you clarify that with them that the price is for a direct ride. If they start saying "oh, it's just one shop" or "I'll take you somewhere better", be firm and say no. Chances are, they'll then leave you alone when they realise you're not an easy mark, and they may even tell you where to go to get a normal auto so they can concentrate on less savvy travellers!
On your first few trips, expect to get ripped off. It's just a rite of passage, and you'll get better over time! Until you know the ballpark figure, you'll pay what seems fair to you based on your taxi experience elsewhere, and most likely way overpay.
Oh, and one final thing, work out beforehand what your personal "close enough" threshold is for negotiating. If you're a backpacker, that may well be rather different to someone well off who's on a work trip, going off somewhere on a weekend or evening. There's no point spending several minutes haggling over USD 0.10 if you've somewhere to be and can afford it, but the difference between $20 and $5 is probably worth arguing over for anyone. Know your own personal limit, and when it's close enough just hop in and get on your way!