Hi Nutty Poster -
First off, it's relatively easy to increase the force of gravity. At least the EFFECTS of the force of gravity (can you tell that physics is one of the things I've majored in over the years?) That would be a centrifuge. NASA and some other places have been working with people in centrifuge for quite some time. Angular velocity (actually not momentum) in a centrifuge sort of mirrors the effects of gravity,
As far as reducing the effects of gravity, the only way, so far, is on the parabolic flights that the "vomit comet" makes. NASA has a couple of big ol' planes that they've stripped down to padded walls on the inside that they use to simulate weightlessness, perhaps even reduced weight, I don't know that much about it.
The plane climbs steeply until it reaches whatever height it reaches. Then it goes over the top like a rollercoaster, then it goes into a powered drive. As the plane goes over the top of the parabola, the effect of gravity is lessened. On the dive down, there are 30-40 seconds of weightlessness.
I've known people who have participated in this - it's not JUST open to NASA people. I went to Purdue, which has a major aerospace department, and Purdue students ride on the vomit comet from time to time for their experiments. I think it's open to other schools
as well. But anyway, I've never been on it... I don't think I'd survive the climbing part of the parabola. I've been told the plane is at a 50 degree attitude (FIFTY DEGREE!!!!).
I just about stroke out when taking off on an American Airlines jet! I Hate that feeling of climbing sharply and believe me I've NEVER been in an airplane 50 degrees nose-high!!!!!!!!
Another VERY unpleasant sounding aspect would be the less than 10 seconds during which you go from 1.8 to ZERO g's. Can you say Urrrrrrppp. Well (light bulb goes on!) I'll bet that's why they call it "vomit comet!"
I work with a lady now who has ridden this several times (doing research for NASA) and said that they do it 32 times per trip! So if you are so miserable you want to die on the first parabola, you know that you have to survive 31 more of them. No getting off.
Hope this helps.