Alot of those designs can be somewhat replicated with just the in game tools. Just have to think outside the box a little. Two unassuming pieces can combine in interesting ways to make entirely new pieces and shapes. That and Kryo's hull mod(KHSM) makes the ship designer almost become it's own ship building game.

First of all as you said, like anything in life, practice is the key to getting better. People who make good ships have probably spent many many hours tinkering in the Shipyard!

Aside from that, I find that it helps to think about structure. Dont just go for form immediately as you may find that you dont have the hardpoints underneath to place outer parts correctly. Turning parts 180 degrees can often give unexpected but useful results.

Basically, experimentation will get you where you want to be and when you feel confident, having an idea before you start, a theme, really helps. Most of the races have a built in theme to me - a simple example is the Thalans whose ships I make look insectoid. If you choose a range of parts that fit that race, then try to reuse those parts in all their ship types, you will end up with a fleet that has a core identity to it and is recognisably the same race.