This is actually really easy.. and kind of suprising that it works..
All you need to do is summon your pet (I normally use the Felhunter).. then go to the Flight Master and fly somewhere..
When your pet disappears it doesnt just give you back 1 soul shard, but it gives you back 2..
Now I basically do this whenever I fly anywhere.. it starts stacking up ..
This one works for me and my bro.. can anyone check if its just a client-side thing or if it really works?