Read these posts I have read alot of people complaining about poor customer experiances, and issues with who is selling them thier phone.
I have been in this feild off and on for about 7 years now, and seen every kind of depravity, from telling a blind man he was signing a $20 a month contract and receiving a $70 a month contract and it goes on from there.
There is no gauranteed protection, but there are some steps that you can take to sheild yourself a bit. I hope some other dealers step up and help with this a bit, but here goes.
Try dealing with someone local - It makes life a little ewasier to eb able to see someone and talk to them face to face, you might have an indication on if this person is out for your interests or thier own. If there is a problem you have someone to go to and try to get resolve.
Sounds to good to be true - It is! Nothing in life is free and noone will ever pay you to take a $400 item in high demand, it doesnt work that way, it never will.
Is the place organized - Cell phone dealers are privy to some very personal information. This is not the stuff you want getting misplaced. If the place is disorganized, if there is personal information laying around, use your better judgement, because your information may be with the rest of information easily accessible to complete strangers.
Look at thier accessory stock - Alot of companies offer "in store rebates" personally these are an indicator to me not to go there, but some people will go for the instore rebate, if you go to a store for an instore rebate and the shelves and displays are empty, thats a good indication that the store may not be there to give you your rebate in 6 months or so. Be perceptive of your surrounding, and use your judgement.
Do they work on commission - If they do then they will benefit from giving what you dont need. In this case be cautious. Commisions arent evil, but if employees are on commission that means that you should look out for yourself. I have worked for many cell phone stores, and the ones that didnt work on commission were the ones that had the track record for honesty and customer interest. They make the same weather you are happy or not, and in the case that you are not happy, you will come back so it is easier for them to make sure you get what you need the first time.
Ask Questiosn - Dont be afraid to ask questions. Sometimes the off the wall questions wont work, but general questions will. You can judge alot about a person from thier answers.
Do they ask you questions - These should include where will you be using your phone (Yeah i know everywhere, but there are some locations you will use the phne more often than others), what plan are you on now, who is your current carrier, are you happy with your current service provider. Any relativly experianced salesperson will know what to ask to speed the process up.
The chicken or the Egg - What do they show you first the service or the phone. Phones are nice, but service is the important thing. An experianced cellular salesman finds out the service that fits your needs first then works from there. I currently have say about 40 display phones in my store, if I select a carrier that does what you want instead of showing you 40 phones I only have to show you 15 give or take. Remember the carrier is always more influential on quality of service than the phone.
Pros and Cons - DDoes the sales person go into the good and the bad or only the good. Every service has its pluses and minuses. Dont be afraid to ask about the services weaknesses.
Do some homework - Internet is alright, but usually biased, or confusing, instead talk to friends. Ask them how they like the service they have, who they use, etc. John Q Public is great to tell you how a phone works in LA but if you are suing the phone in orlando that and 50 cents will get you a bus ride.
These are a couple of factors. They are not all of the tips, but a good start, and they are not constants. These by no means are written in stone, but some of them might make sense to you, some of them are common sense that are just overlooked. I hope they help.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic