You will always have that vibe to some degree, it is likely. Just, use it to your advantage.
Are you a very quiet person? If you are, you'll have trouble because you won't be able to undo the intimidating vibe when the conversation starts, because you're taking the submissive role. That means, the atmosphere of the situation is dependant on them. That means, their impressions are what it hinges upon. If they think you're intimidating, and you're quiet, they're going to treat you like you're intimidating. If you are quiet, I'd say, be very genuine in your *reactions*. Smile a lot, nod. Speak softly, or whatever. Just make sure you listen well and respond positively. They'll very quickly feel a *warm* vibe.
Are you a loud person? If you're loud and extraverted, that's great, because you can use this to your advantage very well. Just impress upon people that you're a big friendly guy - everyone loves a big friendly guy! And then, because you've got that intimidation initially, and then you make them feel good about themselves when you're talking, and they feel very quickly befriended, they'll feel like *they're friends with the tough guy*, and they'll love it. And you'll be the life of the party.
Either way, the idea is to make people feel warm inside and be engaging. Really implore people.
As for eye contact... Eyh. That ain't much of a thing. Try moving your eyebrows upwards, like, very friendly. That's about all I can think of really... But, that only matters if you're actually about to talk to somebody. Those cats that you make eye contact with, they were never gonna come and talk to you anyway. If you're talking about approaching girls, find people who are standing around and aren't intending to leave. Best one, I always say this but, the place is a smoking area. Smokers won't leave until they've finished their cigarette, no matter what freaks are casing. And if it's a smoking area designated for that, then you've got them trapped, basically, hahaha. So, you can make some eye contact, and even if they look away, you can talk to them and it won't be surprising. Smoking is great for that, too, especially if they're in a group - smokers *love* to talk when they're smoking. It's a little upper, and talking becomes part of the ritual. You just need a reason to be in the smoking area, I guess.
In the end uh... Intimidating should work in your favour. You ain't gonna get stabbed, people like to be friends with big guys and a lot of girls like it. When you DO shake their preconceptions people will adore you because people love having their preconceptions shaken. Just be friendly, warm and engaging. Examine the personality traits you would really like to impress upon people and embody them, and tuck them neatly behind your initial intimidation. Good luck =]