HCG is not why you are not keeping your gains.
What does your post-cycle diet and workout routine look like? You shouldn't lose your gains as quickly as you got them when you get done with your cycle. If you do, there is something wrong.
Keeping those gains permanently, however, is a different story.
The flat out best method to keep your gains is to never come off. People can flame away about that all they want, but it is the truth.
If your body's natural hormone level won't support you being 260lbs. then you aren't going to keep it permanently. You may hold onto it for a while, but you will lose it slowly over time, unless you cycle again.
I'm not saying you have to run an agressive cycle year-round, but the "blast and cruise" method works wonders for helping hold on to what you've gained.
Or, as an alternative to that, you may want to run a less agressive cycle and stay that way year-round, for example 500mg/test and 3-400 deca/eq. This way won't swell you up fast but the slower, steadier gains from this method tend to be easier to keep if you do decide to come off.