Orchestration is one of those fields where there is always something to learn, but I thought it would be useful for beginners here to have some concrete goals to start with. As a first goal, aim to orchestrate what I call "CLEANLY".
These are the most basic questions to ask yourself:
1) Is everything I have written reasonably *easily* playable? For your first orchestrations your musicians will not be members of the world's best ensembles. It is ALWAYS better to make things as simple to play as you can. Even with pro orchestras it saves rehearsal time (=$$$).
2) Have I used my ensemble fully? Unlike in a virtual orchestra, adding 4 extra tuba parts is not free in the real world. Adding 4 extra tubas to play 3 notes each means lots of $$$. Is it worth it? Wherever possible, write for STANDARD ensembles, for the same reason.
3) Is the orchestration CLEAR? Is the main line properly emphasized and does it stand out enough in relation to accompanying material? Make the orchestration balance on its own wherever possible.
4) Does the orchestration respect and enhance the form? Making major changes in orchestration in mid-phrase usually will simply distort the music.
5) Are the score and part professionally presented? Nothing gives away an amateur faster than parts badly copied or a score with a wierd ordering of instruments. Standards exist here for a reason: The musician does not have to learn new conventions for each piece. N.B. Having a computer make your parts does NOT guarantee they are OK. Computer generated page turns are sometimes ridiculous, the parts may not be big enough to be read at a distance (remember, the trombone has to be able to see his part at quite a ways off!), etc. etc. ...
There is much, much more, of course, and some points (like #4 above) could be discussed at great length, but I'd say if you can't answer an unequivocal yes to all of the above, you don't DESERVE a real orchestra yet!