This is mostly reiterating what other people have said, but:
Start by bumping up your tax rate. You probably want to keep bumping it up until your morale hits about 55% or so. I say probably because the income from taxes is discontinuous. Sometimes you'll increase taxes by 1% and suddenly you income _drops_ 100 bc or so because a whole bunch of people got upset and decided to leave your empire. However these dropoffs usually happen at much higher tax rates, so I don't think you'll need to worry about it before you hit 55% approval.
Second, unless it's _really_ necessary, even out your spending. Unless there's a drastic problem like a war or racing to reach a really important tech, you should almost always keep the distribution at 33/33/34. There seems to be some kind of inefficincy penalty built in to having the values get too extreme.
Sell some stuff to the AIs. Tech is usually my first choice. If you're on good terms with everyone else in the game, you can get _great_ rates on some of your older military tech. Otherwise you can offload some of the more peacefully techs. Whichever tech you trade, trade it to everyone in the galaxy, unless there's someone you're at war with or really want to hurt. Get the money for it yourself rather than letting some other race decide to trade it around or give it in tribute.
In your case it looks like you've got a lot of military, so maybe you should sell some of that as well, either to allies who have deep pockets or to one of the minor races.
Once you've done all that, start increasing your spending rate. I usually aim to have as small a possible positive balance as possible in the early game (don't want other races getting any ideas about tribute) and start accumulating a little in mid to late game once i'm doing pretty well compared to the others and don't have to worry about tribute. (Well, at least not about _giving_ tribute