I've had quite a lot of difficulty with jumping into high-level characters in a campaign. One of the hardest parts to master is the idea of synergy.
With a long-running campaign, characters grow together as their fitness equations generally include support from each other. That support shapes the direction of growth.
I have three recommendations that I've used to some success. First, try to use essentials characters wherever possible. The simplicity, especially in a one-shot, is very worthwhile.
The second is to create a flowchart or flow-outline. Here is one I made for an epic Battlemind. If possible, have the player who will be playing the character make the notes, because it is the process of articulating the flowchart or edge cases that creates mastery. If that proves difficult due to logistic concerns, feel free to make a flowchart for that person, but encourage them to translate it (in handwriting if possible) before the game into a style that suits their way of thinking.
By thinking through the strategic cases of the character, much of the tacit expertise-building of lower-level play can be abbreviated. By having a simpler character at the outset (try to avoid conditional feats, I find those give me headaches) that is further improved.
The third is to use intrinsic bonuses for items, and limit the character to 3 uncommon items. While this quite satisfactory limits the power level of the character, it also limits the really annoying fiddily bits that items tend to contribute. Static mods of a fire-and-forget variety mean that complex thinking time is not going through the "now what item an I use, and how do I set up that combo" line of thought; a line of thought inappropriate in a one-shot. If a character has a combo, make it easily triggered, visualized, and fit most strategic cases. Morninglord is great. Morninglord, power of the moon, a couple radiant enhancing items, and a holy symbol that sometimes contributes damage... really needs a flowchart which precalculates all the damages in a hash-table lookup.