Same as camerabloke for me, buy a decent laser level as it will pay for depending on how much you're doing! If going underneath windows be make sure you start at the correct height, which a laser level can help. I usually try to put risers behind doors so you can't really see them but if you're working to a drawing the positions will be dictated to you. When using manufactured internal and external bends there is a minimum depth that will accept them both side by side properly, make sure you know this measurement and inform whoever you need to about this, so they can bring boxings around steel etc to the desired depth. I usually get one fixing at the end, offer it up to the laser level at the other end and get it hung, then blast all the fixings in while lifting the middle up as it sags a little bit, I use a chop saw for cutting rather than a hacksaw as this gives a better square cut. There was a bit on the last job we did, although I was slightly hands off due to its size, I couldn't help myself having a bash at it!! It's a job that needs attention paying to it as if it starts to go up or down everything after it will be wrong too and when you get desks in position the differences in gap from the bottom of the dado to top of the desk can look daft.
