Surface Mount Technology

It all depends on the type of circuit you are going to design, but if you want the final result to look really professional and up-to-date, I recommend using surface mount technology. I accidentally received a few of SM transistors when ordering my components for my microphone build, which needless to say I won't be using. (see below)

As you can see from the photo above just how small these guys are. You don't have to make the whole project SM. You can combine SM with through-hole components, as some items are not available (or not suitable) in SM form. You may want to use chips that can be inserted and removed (such as microcontroller chips) or air spaced coils (for high frequency oscillators) or a potentiometer with a knob.