Hi Bradt,
I found getting a bass has made things much easier, though you could use your guitar to play a temporary bass line to start with when you get stuck. If you've got a piece you're quite happy with try playing your bass line through it and carry it on afterwards. This should hopefully mean it will flow pretty well. Always have in mind where you're going to though. So, if you have a verse you will probably want to come back to that after your pre-chorus, chorus, or whatever. You could perhaps put the second verse in your Daw ready - say 8 bars further along or so - then try to play a bass line that gets you from A to B. You'll find you will probably modify this as you go along, but i find it helps to work to some sort of basic idea of structure.
Drums - definitely try and play along to them. I always start with a simple kick and snare beat just looped over and over keeping time whilst i map out what i'm trying to do. You can fancy them up later.
Vocals if you're using them can bring it all together. Try and get some down as early as possible as they may help you shape the music - where you are going to use fills or bits of lead work, etc.
I just try to get a decent rhythm based backing track down, add some vocals, then tart it up with lead lines or whatever.
There's no rules. Just do whatever gets you to where you want to be.
Good luck.
Digger