advice for what to do when it starts... i keep losing it.

...please can someone give me some advice to help me increase these sensations rather than keep losing them ...they do not last very long.

I theorize that the buildup of sensual energy generated during prostate massage creates muscular tension in a manner similar to the muscular tension generated through traditional penile masturbation. Hence, the leg shaking, tremors and other spasmodic muscular actions occur as this energy gets dissipated during orgasm. We learn with traditional penile masturbation we can accelerate the onset of orgasm by adding/increasing tension but this does not seem to work with prostate massage. To the contrary, when you try to increase the sensory stimulation or create additional pressure, this forcing seems to shut down the orgasmic cycle. This effect may be physical, psychological or both.

I believe most veteran users would agree the most effective manner of prolonging the period of orgasmic pleasure waves is to relax, please see @darwin's post relax, relax, RELAX for a little more insight. So my advice to you, when you feel ..."it"... beginning, is just try to relax into your sensations rather than trying to "... increase these sensations..." by harder contractions, manual manipulation, supplemental vibrations, etc. Your body will only generate the sensations it wants to generate, not the sensations your ego wants it to generate.
Good Vibes to You !

When I feel "it" coming on I consciously just clear my mind and kind of "fall away" from my body. Kind of like I completely surrender control of my body over to the sensation. If I'm lying on my back it's like my consciousness disconnect from my physical body and I start falling back away from it.

I guess getting your brain out of it as much as possible it beneficial.

Have Fun!

Thanks both, i should have said I have been trying the relax method and it is the one new thing I have added lately that got me to the nice feelings and leg shaking.

Problem is I think it shocks me a bit and that derails me... then I can sometimes find it hard to get back to that place.

@Marco_Polo I find that the hardest things in prostate massage with Aneros are determining "when" to let-go of control (it is harder to do than to say it) and when to stop the session. Not every session is going to result in a Super-O! But it should be stimulating and sensual.

Problem is I think it shocks me a bit and that derails me.

This is definitely a recurring problem for me. When I feel it start to build it immediately draws my attention and kills my momentum.