Build more units, and upgrade them as soon as possible. If the AI perceives you was 'weak', then you'll spend a lot of time in war. If it perceives you as 'strong', you won't.
So build more cities, road all your tiles in the city square, build libraries and uni's, trade techs with the AI as much as possible, and get the resources before the AI does. If you're sitting pretty with a bunch of infantry, and the best the AI can do is guerillas, it won't attack unless it has a lot more units than you.
And for the record, the AI gets zero combat bonus'. The only bonus' that apply are against barbs. I don't recall what they are, but at Warlord the AI/human as about a 150% better unit than if against a non-barb unit. So at the lower levels, you have quite an avantage against barbs. But the AI gets no bonus' to attack. Period. Now, in the higher levels it does start out with more units, but they're not stronger. . .