Avoid using the techs that reduce planet quality, unless you KNOW you're going to lose. If you're invading a 20b planet with 1b troops, use mass drivers to maximize the casualties - the planet won't lose PQ unless you win the invasion.

If you have higher soldiering than your opponent, use minisoldiers if available. It improves the advantage you already have, and does minimal damage.

If the enemy has higher soldiering than you, use tidal disruptors. Yes, you will lose much of what's on the planet, but it cuts their advantage over you more than minisoldiers boosts your advantage. Absolutely devastating against Dread Lords (not that you really need it, but whatever).

If the enemy has low morale, (or can be made to have low morale with spies in DA/TA) use information warfare. It's pretty easy to end an invasion with more troops than you had on your transport. It is expensive, but you'll make much of that up by not losing buildings to tidal disruptors and not having a really low population planet.

Thanks guys I'll take this into account in my next invasion!!!

That still works in TA. Just be careful if you have an extremely high soldiering stat - I have ruined planets because I won at 12 to 1 odds, even 14-16 to 1. Don't try this if the odds are less than 10 to 1, unless you are badly out-teched.

I've been giving fleeted transports another try, and they are quite effective in some circumstances. You always lose at least one transport (just like attacking singly) but any transports you can fill with the surviving troops, you keep. (attack with 9 1b transports and live with 7.9b troops, you would keep 7 transports; win with 12+b troops, you would have 8, not 9). This is most effective with info warfare and mini soldiers (or tidal disruptors if you don't have the tech to use their buildings). Only use it when you have a pretty big advantage though - close battles are still better off using sequential invasions.

I for one like to use a somewhat more advanced tactic.

apparently if you have a lot of troops your enemy's advantage suffers. SO I create a massive fleet of troop carriers and bum rush a planet with them.

we're talk 16000 or so troops versus 8000 here, for some reason I've seen the enemies advantage halve.

Maybe I'm just being a newb and seeing a number comparison gauge that I normally don't see because blackr ace colour, on blace race background is a bad idea?

Are you saying you attack with 16000 troops or 8000 listed?

Either way, halving advantage or not, that just isn't viable as part of a larger campaign. You'd be tooling around for hundreds of turns before taking out a single enemy capable of putting up a fight.