[dmm]

In my experience, it seems to help the AI to start with multiple planets. (For a REAL challenge, give everyone five or even ten planets, then abandon all but one of yours at the start.) It also helps the AI to expand if you give it special abilities like movement bonus, supply use bonus, and natural merchant.

If you like to roleplay, you can add a rule for yourself that you must gift or trade tech to everyone after a set number of years. This simulates the real-world difficulty of "bottling up" your technology. This especially applies to commercially applicable stuff like ship size, engines, resource production, etc. Of course, in real life some techs are more tightly controlled than others, e.g., stealth or star destruction, so maybe you'd have separate categories. (Wouldn't it be cool if the computer kept track of this for you, and made it easier to research stuff that you'd seen on other players' ships and planets?)