1.  Colonize the star systems further out first, then back-fill.  You can get a lot more star systems that way.  If your aren't colonizing star systems that are within or near an AI border, then you're doing it wrong.  

2.  Get ion engines.  Put two of them on your colony ships.  Get rid of the life support module. Your colony ship costs about the same, but moves a lot faster.   

3.  Extreme colonization (for the planets they couldn't take) + influence starbases +influence techs.  Who says you have to fight them? 

4.  Bribe someone else to attack them.  

That is true that U can a colony ship out and build a colony ship from the planet and colonize nearby I would do both methods send out colony ships and colonize fro m there all I can say is who R your neighbors like the Thalans Torians the mody the Korath the terrans The Krynn might out colonize but if it is one of the others U can colonize around them preferabl y sh but the yor drath dengin or not my version of the Korath R neighbors Do U have any BC left U can rusheconom research military production keep your economy high trade for economic treaties keep a high population build about four factories on a planet R U reakkt desporate U can do two things focus on ween research anmilitary production on the planets but consider the conquences if U do this U lose more betwioneen social productionry  and research than U gain on military production U can also up the militariony production on the civilization screen also check out my stradegy guide it will help U can also build economy and be smarter with your space miner

You let them be friends? Are ye daft!?

All kidding aside, letting the AI set up friendly relations is unwise in almost all circumstances. Whenever you have contacted AI, try to get them to war one another unless it is very inconvenient for YOU if they do. If they fight eachother, they will waste precious resources not fighting you and are also less likely to attack you, which gives you more time and space to dictate events. As Julius Ceasar said "Divide and Conquer".

Now, if that tactic has somehow failed, there are some strategic pointers: You do not need your fleet to be on all fronts at once. The most important insight is that galciv wars aren't won in space, but they're won on the ground. If your transports are fast enough and you can sneak a fleet in to kill the defenders around poorly defended planets, you can gain territory in spite of enemy fleet supremacy. The AI isn't very good at taking planets, so just pick off easy targets if your fleet is too weak to fight the enemy in space, keep transports standing by to immediatly reclaim lost planets and generally all will be well. Just remember, if you can't be stronger, you should be faster!