I've spoken in a few prior entries about the importance of negotiating a scene beforehand, so I think it's about time I expand on something so paramount.
There are a fair amount of either "no limits" subs out there or guys who are so submissive they want to be submissive even in a scene negotiation, and this information is not intended for them. This is intended for men who want to ensure they have a scene as intense as they are hoping for while protecting themself from being taken too far or otherwise hurt. Obviously there are too many potential red flags and precautions to generate an exhaustive list, but here are a few starting points:
Libido is for scenes, not for the discussions leading up to them. This is the single most important thing I can say regarding this matter. If either person involved is horny enough to be jerking it while discussing a scene, you should take a break and talk again when you're both feeling more level-headed. This is imporant because intense horniness is not present every moment of every day, nor is it present every moment of a scene. If you discuss things in that state, odds are if the energy isn't there the interest won't be either and things will fall short of your expectations. If you discuss things when level-headed, you're much more likely to have a scene that's an overall success since the energy required to be interested is sustainable.
Expect patience to some degree. Given the level of trust required for most BDSM-related play, impatience should set off some serious alarms. Obviously everyone will have limits as to how much negotiating they can put up with, but if someone is trying to push you into playing before you're comfortable you should take a step back. Be honest with yourself and see if the issues you had brought up are things you legitimately feel or excuses to delay things. If it's the latter, either man up or take some time off to do some self-evaluation.
Keep a poker face. Horny guys will lie until they get what they want. While you definitely want to have something showing roughly what you're into, don't lay out an entire, detailed scene for a guy. Getting input from him without knowing the way you approach scenes lets you determine whether or not the guy has the right attitude for you without deliberately sneaking in things he knows you'll find are hot to distract you. Everyone has different styles; some Doms are stoic, others are very verbal, some emphasise abuse, others focus on service. Letting the Dom show you what his strong points can help determine his priorities.
Ask for references or proof of prior scenes. Whether it's pictures of guys someone has tied/dominated or first-hand accounts from people a guy has played with, it never hurts to have more than a man's word. If you're going by pictures, look for common factors; similar ropework styling, similar playspace, similar gear, etc. If one picture has a basic 4-point spreadeagle and another has an intriciate, shibari-style hogtie don't just assume he was lazy one day.
Determine how the scene will begin. It sounds self-apparent, but sometimes it's easy to forget. If you both have different expectations of how the scene is to begin, it can throw things significantly out-of-whack. Some guys want to relax and chat prior to a scene to get comfortable, others may want to skip straight to submitting, and there's always the trusty "instructions at the door" scene. Point being, a scene takes some degree of mental preperation and starting off on the wrong foot can undo the entire process.
So hopefully this should be a good starting point for anyone who isn't having good luck finding the scenes that they need. Just remember that it takes a lot of patience to find the right guy. If the discussions aren't working out don't just get impatient and go with it; keep looking until someone hits the nail a bit more squarely on the head.