1) When do you all switch governments from the default despotism?
I usually do it ASAP, unless I have the Great Library almost done, and when I do, it's almost always to Republic(ok, one game where i went to monarchy

)
2) How long do you take before you start a big war?
I'll start a war if someone has something I want and I think i can take them(in one of my games i started over on, I would have been boxed in with maybe 4-5 cities in ICS sprawl, but with a quick war against Tokugawa, I was able to take hold of half the continent, as well as get many a tech from him. And I was able to banish him to the tundras of the North). In a nutshell, if they have a monopoly of anything, and if I can take them on without losing too much(like a city...), I will take it
On the larger scale, the World Wars are usually in the industrial age, when I know who should fight against who(and after Fascism is discovered), I'll save up lots of money and bribe as many civs as I can to start wailing on other civs(usually weaker ones but ones that can still fight).
3) In a game *(currently playing) I have the largest civilization by both pop and territories - is this good means for warmongering or teching? I'm playing as the celts.
I've found that how big you are doesn't necessarily mean you're going to be able to out-research the AI. I had about 30% of the world in my game, and found that I was 3 techs(!!) behind two of the AIs. Granted they were Modern Era techs, but still. They only had about 20% apiece of world area/pop. The point I'm trying to make is that you can't research everything by yourself(as I'm trying to get out of the habit of), so for individually getting techs, being big helps a lot, but you'll still have to break down and start trading.
As for warmongering, being big is great(especially with Communism). For governments besides Communism, the logistics can get a bit messy(having to wait between 5 and 10-20 turns to move a tank from one city to another across the world to for war(non-pangea). But that all changes with Airfields. I have my core producing almost EVERYTHING and have between 5 and 10 airfields on each island/continent. This saved a LOT of turns in my last war, especially when TOW Infantry was discovered by the AI after I had already declared war. Now I could send in fresh troops to start fighting continuously, with only having to wait 1 turn.
Logistics aside, being big is great
4) What are your views on improvements - should I be building all of them or are certain better at certain times?
In one of my favorite games, I made the mistake of not building enough culture on my border cities, and thus lost out of two very important resources-rubber and uranium. I'm going to say build them around your border, and when your border moves(unless the city isn't corrupt and can get use out of the libraries or temples), sell them as the new(-ly conquered) builds up its cultural border. Had I done this, I wouldn't have had to go against the Mongols and Aztecs for the resources Hiawatha "took" from me
5) How should I fix my lux/science sliders? Should I be trying to keep up with the AI's (they're a little ahead I think by 2-3 techs), or should I be getting a lot of GPT?
I don't think you should be getting very much gpt(except for techs ending in less than 4 turns). I usually have the research as high as it can go without upsetting my happiness slider, and start lowering the science until i am starting to get the lowest amount of turns(ie if I have 4 turns at 90% research, I'll lower the percentage as much as I can to still get 4 turns. Be sure to check on it, and when it gets down to 2, 3 or 4 turns for a tech, you can probably slack it a little, but not so you're going to be getting it in 3, 4, or 5 turns). I am also usually losing money(between 8-20 gpt), but usually make it up by doing what I said before, so it evens out, as well as setting research to 0 with the GL built(LOTS of money) usually. And remember not to give the AI any gpt, but you should be getting gpt from THEM

muck up their research as much as you can!
Anyways, thanks for the help.[/QUOTE]
Note: Most of the advice is on Monarch level. I personally have the problem of not having enough troops(or so my advisor tells me), but i find it's helpful to have LOTS of cities in ICS on tundra and desert, keeping the grasslands somewhat open, and if you have a city that has all hills/mountains/grasslands, put the forbidden Palace THERE(That's going to be a LOT of production, and it might have Ironworks...

). And finally, always be building troops and get Great Library, Leo's Workshop, Smith's Trading, and Theory of Evolution for sure! Those are about all of the wonders I built in the game, and I'm doing all right for myself. It really bites if the AI gets one of them...
Michael