This is, as you said, quite a big question. Here are some of my thoughts:
Developing Environment
First of all when I start a new project I install Sitecore on my developing environment and I make sure everything works. Either during installation or after I place the databases on a separate SQL-server and change the connectionstring accordingly.
I open up Visual Studio and create a solution and include the files needed. I create some kind of HelloWorld rendering and try building the solution so that I can verify that everything is working as it should.
When everything is up and running I create a zip-file of the whole solution, including the data-folder. Now it is time to add this to some kind of version control system, in my case Subversion.
I add the zip-file to subversion and also add all files that I think will be changed during the project, usually I tell subversion to ignore the sitecore folder, this speeds up performance drastically when checking in files.
After I perform a commit-action the other team members of my project can check out the code and start developing (after unzipping the zip-file, off-course)
We all work towards the same database although this goes against Sitecore recommendations, we havent had any problems with this approach however items in GUI created/changed by one developer take some time before it is created/changed for all the others.
We could off-course develop several different projects using the same Sitecore installation but since almost all customers use different versions of Sitecore we have found this approach a bit cumbersome.
Often we set up an automated build-server but this is a whole other issue.
Reusable code and renderings
I would like to say that we create neat packages based on the same codebase that gets reused between projects but unfortunately we are not there yet. Today it is a lot of cut and pasting between solutions.
Uploading code to customer
This is done via sitecore packages, normally with some kind of dynamic selection for what files to include, say all ascx-files in a specific folder changed the last 5 days.
There you have it.