From what I have seen, "keepable" gains come down to being mindful of 2 issues:
Water bloat
Possible catabolism after a cycle ends.
Now, I am not knowledgebale enough to get into all the technical BS on this but from what I have read ....
Some AS tend to promote a LOT of water retention. Now these tend to be many of the AS's that give "fast, massive gains". Stuff like D-Bol for example. DECA also makes you hold water. Test can abd does, but it is dependant a lot on each person, the dose, whther or not anti-Es are used, etc.
The thing with this sorta bloated, water-based gain is that, ....... weli ...... in my opinion they werent gains to begin with. Its just water. Sure, you get HYOOGE but its water.
This effect canb be lessened by taking a good Anti-E like Arimidex.
Now, to handle post-cycle, it imprtant to come OFF the cycle correctly. Time things right. Take Clomid, take and Anti-E. Eat well. Train ..... enough to keep gains, but not so much as to overtrain and go cxatabolic (and rememebr, that means your weights and intensity WILL be dropping as the AS's get out of your system).
The peolel that I have talked to that LOSE gains, tend to either/or have made a LOT of gain which was just retained water, or they come off the cycle wrong, and mess things up.
That's why for me, I selected EQ instead of DECA to stack with Test. I also have done a low anti-E dose throughout the cycle, so that I would not bloat up. I have grown, and for the most part it has been hard, solid gains all the way.
I will come off in a few weeks and am ready for that as well.