I like the Pierce to Dallas idea, but I'm not sure if they have to wait until the first of July to trade Marion.
What about a three-way trade with OKC to help Houston clear space?
Celtics out: Paul Pierce
Celtics in: Perk, Thomas Robinson, Royce White, Perry Jones and this year's #12
Houston out: Thomas Robinson, Royce White, Omer Asik
Houston in: Paul Pierce
OKC out: Perk, Perry Jones, #12 pick
OKC in: Omer Asik
Why for the Cs? Get a lottery pick, get interesting prospects in Robinson and Jones. Of course, it means carrying Perk's horrid contract and dealing with Royce White.
Why for Houston? They get Pierce (clearly the best player in the trade and expiring contract) and they can keep him or buy him out to go after Howard/CP3. Give Asik and T-Rob, but they get rid of Royce White.
Why for OKC? They get an upgrade in Asik for just Perk's money, so giving up Jones and the #12 pick could work out just fine.
After this, Houston could become an even bigger threat for the Clips, so you can force them to make a KG/Bass/Terry for Jordan/Butler, or even better, the three-way proposed by CroCorvus, but expanded like this:
Boston in: Cousins/Salmons/Butler
Boston out: KG/Bass/Terry/Lee
Clips in: KG/Bass/Terry
Clips out: Jordan/Butler and the #25
Kings in: Jordan/Lee amd the #25
Kings out: Cousins/Salmons
Celtics team without the draft picks (#12 and #16):
Rondo(2yr)/TWill(exp)
Bradley(exp)/Salmons(2yr)
Green(3yr)/Butler(exp)/Perry Jones(3yr)
Sully(3yr)/T-Rob(3yr)/White(3yr)
Cousins(exp)/Perk(2yr)/Melo(3yr)
You can draft Adetokunbo with #16, and stash him in Europe for 2-3 years. Use OKC's pick to get Shabazz if available, and find a veteran PG in free agency for limited money.
Basically, you need to extend Bradley (4yr/15M) and Cousins (

?), move Butler at the deadline or let him walk, and carry Salmons and Perk (both probably can help off the bench) until next season, when Rondo is due for an extension.
Do it, Danny!