Try building more gold mines. Make sure you have at least 6-7 peasants going into each gold mine, depending on the distance from your town hall. Your lumber mills should be able to get you by with only a handful of workers chopping trees. You'll be able to afford the upgrades to your knights in no time!

If you're playing Sins of a Solar Empire and not Warcraft, however, I'd recommend making sure all your planet's are fully upgraded (more people = more taxes) and to start building trade centers. Selling excess metal is a good way to earn quick income as well (research the +metal technologies and you'll be getting an abundance very easily, very early).

There's one of the single player maps that gives me a problem... Its the one called 'Fertile Crescent' or something like that. You start out with a planet earning you -2.2 income. If you blow your starting funds on the necessary upgrades to avoid this fate, you wind up watching your credits inch up for like an hour. Any ideas?

Here's a tip nobody's mentioned yet

Once your empire gets large enough, move your homeworld closer to the center - that way the majority of your planets will be closer to it. This will increase max allegiance on a lot of planets - and thus give you a very considerable boost to income.

Especially do this if you're expanding into another solar system.. star jumps count as a normal jump for purposes of allegiance