Permanently, and Forever, A Tree.
Get a high level Wu Jen to cast Arboreal TransformationArboreal Transformation on you.
Use the Psionic SandwichPsionic Sandwich trick, but on a tree.
Use Polymorph Any Object to change yourself into a Bush, and then have someone else use Polymorph Any Object to change you into a Tree. (it checks for duration at cast, original polymorph wears off but doesn't matter, as you are still permanently a tree)
Cover yourself in quintessence and have someone grow a tree around you, Han Solo Frozen In Carbonite style.
Have someone lay down a Energy Transformation Field, key it to an Ocular Tree Shape, and mindrape an automaton (Warforged work well for this), bury it beneath the ground with an Immovable Rod, instruct it to keep pressing the button forever. Stand where the Ocular Spell is aimed. BOOM. Instant and forever tree.
Be a Tree, get turned into a human via Polymorph Any Object. Gain levels, go adventuring, and then get dispelled. BOOM TREE.
Anyone else has any ideas, I will add.