I wouldn't say you would have a long term negative effect, although it does depend on the drug/dose and what you define a bad trip to be.
For example, would you say that a trip is bad because of something buried deep in your subconscious being brought to the surface? This could be a negative memory and dampen the trip, but it's all subjective, because you may decide afterwards that it was important for you to know about it properly, and enable you to do something about it.
Or perhaps you mean the fear? Being confused? Perhaps being overwhelmed? etc? Again, this all comes down to how you handle it during and after.
Either way, you are pretty unlikely to have a bad trip, unless you treat the drugs like a recreational substance like coke or mandy. The trick is all set and setting, so be somewhere you know or can cope with, and with people you trust.
Dosage wise, if you are new to psyches, I recommend low doses just so you begin to realise what you are dealing with. In terms of more precise doses, I'd recommend 1g of dried mushrooms, 100ug tab of LSD, or 200mg of mescaline. You should be able to experience the more psychedelic side without getting locked into anything too overwhelming that way. Just be sure that you aren't going to be interrupted, that you are in a safe environment with people you like, and that you have the day after to yourself, and you should be fine.
Sitters are there are a preference for whoever is taking the drug. I've never had one or wanted one, but if you are anxious, it may help calm your nerves. Whatever you do, don't get a sitter who knows jack about psychedelics - they will likely only make matters worse should you have a bad trip.
Be positive going in, and have a reason for why you want the experience. Remember at all times that it is just a drug, and should you enter a bad phase, know that you will always come down. You can even write "I will come down" on your hand, but I'd consider this overkill, and worrying about it is likely to make it manifest itself.
There's also the 100% guaranteed way of ensuring a good time - do MDMA with the dose. Psyches do play on your state of mind, so if you are in happy la la land due to the MDMA, your trip will be as well.
Best thing you can do is to relax when you take the dose, keep the conversation upbeat, and listen to music you love. You'll soon realise there's nothing to fear.
With regards to DMT, bad trips are generally made worse by the fact that you can't get off. Given that a small dose of DMT (0.02mg-0.03mg) will only last 5-10 minutes, you won't have this worry. If you find it uncomfortable, just kick back and relax as much as you can as it will be over very quickly anyway.
Life should not be a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside in a cloud of smoke, thoroughly used up, totally worn out, and loudly proclaiming "Wow! What a Ride!”
― Hunter S. Thompson