I guess first you've got to define what "non-tanking" means, since it's hard to come to a consensus around these parts what it actually means to tank.
I'll assume you mean "Rebuild without being bad enough to get a top 5 pick."
If we're following that plan, I think the best we can do is
- just try to make the best of our late lottery / mid-1st round picks,
- avoid overspending on role players (our own and on other teams),
- look for guys in free agency or via trade who we can buy low, build up, and sell high, and then
- try to position ourselves to make a couple of big trades for key players to lead the next Celtics contender.
Combine that with
- some quality draft picks and
- a couple of smart free agency additions
and we could have a dark horse contender along the lines of the Pacers, Warriors, or Grizzlies.
Our team won't be in the upper echelon inhabited by those teams with the true megastars like LeBron and Durant, but it'll be entertaining and competitive.