Many times the boxy sound comes about because you are actually singing in a box - the room you record in.

If you have a condenser mic, their polar pattern is much more sensitive to the sides, all sides, including the ceiling.

I would need to hear your recordings to be able to tell if this is the culprit - it often is. Also, make sure you high-pass your vocals starting with the mic if you have a switch; enable it. Then play around with the cutoff of a high pass filter on your recorded vocals starting at around 100Hz and going up from there until it seems it's just 'thin', then back it off from there.

-Scott