I like to use simulators because they often have lots of models already defined and all you have to do is choose what part you want to use, or look it up on the web and use that.
It's also nice to do some of this by 'hand' too, by calculating various things about the circuit and looking at it theoretically, and also using some form of numerical analysis which can be very interesting in itself and is much simpler than having to do lots of algebra.