I've not done much, but what small experience I've had with twixtor-type programs would indicate that shooting at a high shutter speed helps a lot. also, shoot at the highest progressive fps possible (not actually sure what this is on GH2, I'm a 24p man myself)

Renting a Phantom in Australia means getting the operator as well... which is about 9-10 GH2 worth of cash.
+ if you are filming inside (don't). Otherwise at 1/1000 sec shutter you will need 20k + rental of lights...

Besides - slow motion is so 2011! :-P

MVTools 2 is comparable to the standard version Twixtor (sometimes better, sometimes worse) though it doesn't have the features of Twixtor Advanced. It is also typically quicker to render. So if all you want is slow motion post-processing, that's another good way to go and it's free (just a bit harder to learn). The main thing you give up, is the easy timeline method of changing playback rate by rendering extra frames (which Twixtor supports).

Getting back to shooting, these programs all have a much easier time with a narrow shutter and higher fps. Honestly, you'll get better results slow-mo processing 720 60P at 1/4000 than 1080 24P at 1/4000. That means that getting 240 fps out of 60 fps is easier than getting 96 fps out of 24 fps, because it has to do with the clarity of the original image and the difference between the two.

@B3Guy already touched on the issue of shutter speed, but let me mention something I find helpful. Apply the 180 degree rule to the frame rate you want to have AFTER you process the footage. So if you want 240 fps, you should be shooting with a shutter speed of 1/480 or less (1/500 being the closest on the GH2). Even if you are using 24H 1080 24P instead of SH 720 60P mode, still apply this.

Pixel motion software like Twixtor and MVTools 2 do better with objects that move less from frame to frame than ones with large movement, so this into account as well. If you need large movement, you'll generally have to avoid processing with pixel motion software and rely on the frames shot.

If you are shooting rushing water, specifically, 1/500 (there's that number again) should be the slowest shutter speed you use (at least that's been the case every time I've shot). 1/250 and slower than that is MUCH more difficult to process for MVTools 2 and Twixtor.

Another trick, in terms of dealing with things moving quickly, is to shoot wider or from further back. That way the motion goes across fewer pixels and is easier for the algorithm to track.

I spent 200 clips comparing different settings, lenses, positions, frame rates, etc. on water specifically recently and everything I'm saying held up through that experience (and some of it originated there). If your own experience diverges, please let me know because I'm always happy to test and verify such things. :)

Well, you don't need to rent a Phantom any more for most purposes, if an EPIC goes to 300 fps in 2K.

But if you need artificial slo-mo, everything thepalalias wrote is identical with my experiences (lots of dance theatre). No spontaneous after-the-fact slo-mo in good quality, sorry.

An interesting way of research can be the 72 real fps (3*24fps) achieved by mpgxsvcd settings (see page 1 of "PTool v3.63d topic").

"72 FPS works. It indicates it is 72 FPS and it looks like 72 FPS. Not sure if 72 FPS is a good thing or not though? It looks weird and 1/160 of a second shutter speed looks bad with the lights I have near me. Trying 120 FPS now."

@mpgxsvcd : Hi! What settings have you use for reach this result?

ps: excuse my poor english ^^