I will give you a few tips and assumptions through my own experience, which happened a few days ago.
I was at a work interview for a part time position, and I know I had to be very positive and energetic to keep their attention and really stand out from everyone. The job is a very customer orientated role, so very enthusiastic communication gets you far. So for my job interview I was very PUMPED, I guess you can say I HAD to get pumped, at first I was faking my enthusiasm, I wasn't very enthusiastic deep down, but after about a minute it's like I forgot that I was faking my enthusiasm throughout the interview and really just started to build energy. The energy kind built up like an avalanche, the more I kept talking and engaging the interviewers the more charisma I built and the more energy I got.
After my interview I was so pumped I went and opened a few sets during the day (and I DON'T really do daygame, I don't have that much experience), had I not built up that charisma and energy during the interview I don't think I would've had the guts to open (and certainly not the guts to open in the manner to which I did).

So I guess you can try fake your state, till you make your state. Go out next time and randomly talk to some girls with no intention of trying to game them, just tell them an exciting story and let your energy build from there. People are different, some people might take a minute to get in a positive state, and some people may take an hour.
If you believe you are in a positive state, then you ARE in a positive state. If you think, oh no...I'm too negative right now and girls will notice, then you are being negative..

EDIT: To add onto this (I just remembered), apparently the body lets out a chemical which creates energy (or the chemical IS energy), you would tend to be MUCH more energetic if you had just gone sky diving than if you had just come back from the library after reading a newspaper.
So maybe you can try doing some more energetic things before going out? (I'm not suggesting skydiving, but use your imagination). Please note that apparently depending on the levels of energy created, it will go down in time. Depending on the levels of energy released in your body.

Ya it's a hormone called adrenaline.