How to trim the fat depends on what the fat is....
If building expenses are too high, considering selling libraries in hopelessly corrupt cities. They do essentially no good. Actually, unless you're researching fairly heavily, they do very little good in ANY city, so they could/should be sold.
If corruption is a killer, check your courthouses/FP location/etc. See if you can fix it.
Is the base number too low? You need more roads. And/or your cities/towns aren't growing fast enough, so that your total population is too low.
Are unit upkeeps killing you? Disband some military or go bonk some more heads.
Do you have specialists? You can often make more gold by actually turning up the luxury slider. Are you using it efficiently?
A save file is always the ultimate in getting good advise, but even a picture of your F1 screen can be illuminating (F11 is also a good place to look). There are a number of possibilities, but it's hard to evaluate which is true from so little information.
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Temples have their effect (increasing total culture, 1 happy face) forever, yes. Their culture output even doubles after 1000 years. Even in totally corrupt outlying cities, they're probably worth keeping.
Libraries increase science and produce culture for their entire life, too, but unless you're getting actual commerce benefit from them (e.g. you're running reasonable science and the city isn't terminally corrupt), they're probably not worth keeping. If you're currently running low science but plan to crank it back up again at some point, libraries are worth keeping in core cities.
Colosseums are rarely worth building. Cathedrals depends on the happiness need of your civ, which depends on how many luxes you've been able to accrue. Sometimes they're worth it and sometimes not. IMO, one of the beautiful things about Civ3 is that hard-and-fast rules are rarely correct. You have to adapt to the circumstances of the individual game.
Arathorn