A lot of people seem to have serious trust issues around here.
At the end of the day, a car that's been maintained well by a reputable garage with all the receipts is going to be more appealing to a buyer then one thats been serviced DIY.
Yes there are plenty of places out there that will rip you off but there are also plenty of places who are in it to make an honest living.
I can do a lot of DIY stuff myself, brake disks etc but I wont get into suspension bits and the like. I expect I'm not alone in my level of ability so I'll go to garages to have work done.
Avoiding getting ripped off is about common sense - agree in advance what the garage is doing and at what cost, make it clear to them that if they feel anything needs to change they should contact you first to confirm or you wont be paying. If they say something else needs done, challenge it if you dont believe it - go back and let them show you what the problem is. If you get there and they have done more than you agreed to, dont pay it - tell them to undo it if they have to. I had a situation like that a little while ago with my old Seat, it was in for a service and a few bits doing and they had replaced a handbrake cable without telling me. I knew the garage was a good one as I had used them before, as have plenty of friends etc - they ran me through the bill and when they got to the handbrake cable I told them I didn't agree to having that done. The manager apologised, said they were going to call but had obviously forgotten so waived the charge for that part of the bill
If you then find out they've done a bad job, you take it up with them, demand they sort it and don't go back to them.
And stop crying "oh noes i paid £100 to have a £25 part fitted!!!1!!" - you're not paying for the part, you're paying for someone's time and the garage's costs to do the job professionally. If you think you can source the part cheaper, ask them for a breakdown of the parts/labour costs before you agree and buy the part separately if you want, no garage is going to have a problem with that.
If you do your homework (taking advice from other people's experience, looking on the good garage scheme site etc), use your common sense (as above) and dont treat them like they're going to rip you off, you significantly lower the chances of being conned
Edit - oh, and try to avoid the big chains where possible. They type of staff they attract and the sales techniques they are told to employ mean they are more likely to try and rip you off than an independent. Some are good, some are bad - I know my local nationwide is pretty decent for example