I'm working on a 5+ year old ASP.NET project that has 74+ projects and we've been adding new Silverlight applications to run in the ASP.NET page islands. My machine at work isn't the most powerful, so I find myself waiting a lot for the whole thing to build. I'm using Visual Studio 2010, so that takes up a lot of resources as well. This causes me to get distracted and I start looking at the news... I need to combat that more :-). I can't get a new machine, that's up to someone else, so I've found a few tricks to help. 1. Only build the Silverlight project you're working with. This will build all referenced projects (you can see these by right clicking and clicking Project Dependencies) and package a new XAP (you can see all the actions in your output build window). Then refresh your page with the Silverlight app and it's up-to-date. 2. I was working with a co-worker (thanks Jordan) who was using the the Debug -> attach to processes window. In the Attach to: row there is a "Select..." button. In the dialog, click "Debug these code types:" and select Silverlight. Hit ok.
Then all you need to do is find your process (you might need to click the refresh button). I'm usually debugging in IE, so I select the first one and push "i" on the keyboard. That brings me to the IE windows open. Find the one with type of Silverlight, x86. It is usually directly above one with type of x86 that has the page title for "title". Click attach and watch your output window spit out messages about loading debug symbols and your breakpoints enabled (if this doesn't happen you chose the wrong process, hit stop and try again). Now you can debug the client code as normal, server code requires a full F5 or attaching to the correct process.

To improve this even further, bind the menu item to a key stroke. I chose ctrl + x, x. (Tools -> Options -> Keyboard, search for Debug.AttachToProcess, set the shortcut keys globaly and assign). Most of the time I build the project, then hit ctrl + x, x then i, then enter and I'm debugging. The process I want is usually the first IE in the list.