How big is the tree ?. If it's still fairly small, you will have a better chance of retaining the typical conical shape.
The height can be checked by taking out the leading shoot at a branch junction, they do this on Christmas tree plantations using a long knife, sometimes one in each hand and slashing down each side of the tree, this keeps them compact and bushy.
Don't be tempted to go in too heavy, just about 1/3rd of the overall length.
Best time of year, I think would be late spring, this will allow pruning cuts to harden off before the winter.
I've seen some very odd shaped ones, done by people trying to keep them small
Good Luck
Chip