Have someone hold the post plumb and level for you. Then take 1 2x4 and nail it to one side of the post at a 45 degree angle to the post. (Let the 2x4 rest on the ground.) Then take a 2nd 2x4 and nail it to a side of the post that's PERPENDICULAR to the first 2x4, it too, at a 45 degree angle to the post. Then, you'll need 2 stakes to nail each 2x4 to.
Have your friend level and plumb the post again. Set a stake in the ground where the 2x4 'rests' on the ground, and nail them together. Do the same thing with the 2nd 2x4. Re-check your plumb and level. 'Move' the post or the stakes as necessary. Pound the dirt around the stakes when everything lines up. Done! Of course, when you pour your concrete or backfill your dirt, be careful not to knock the post out of alignment. Keep the level handy. You don't need framing nails!!! Use 3" finishing nails, and don't nail them into the post all the way. If you do, you'll have no fun getting them out. Pre-nail the 2x4's before you set them up against the post. Pre-nail into the stakes before you nail to the 2x4. You may need an 8' 2x4, then again, you may only need 6'. Don't CUT an 8' 2x4 to 6' if that's all you need. Just nail at the 6 foot 'mark' on the 2x4. The shorter the post, the shorter the 2x4 you'll need.