Fast pipeline production

me as a level designer, if I have to wait for you to spend 2000 hours on your highpoly before getting the object in game...

time = nothing

time til' shippable = as long as stuff allows on project

time til' final = never

edit*

you need to know when to stop, at all stages, be it a placeholder, milestone delivery or "final". Big picture is all that matters. How does your object look in an environment?

1.does it pop out in a bad way, fix it.

2.does it pop out, in a good way, spend more time on it (if you don't have too many of case #1)

EDIT: Re-read OP so here's so more tips specific to asset creation:

Add writing to your sketches write exactly what you need to know when modeling such as what material is on what part

Get the model in game NOW. so you've got a primitive roughly the size of whatever you are making and youve extruded a few polys and it's VERY early. put it in game, that way the name of the asset and the windows you need to use are ready to go for your next iteration. also helps spot problems and avoids fatigue (as in, "I just spent eight hours on this model and I don't want to do the tedious stuff"

U/V'ing woes? Learn pelt mapping pelt map all but the simplest models then work on the u/v from there. u/v is like anything else, you should do it in stages starting rough and getting progressivley finer

U/V NOW U/V parts as you make them. don't worry about layout, size, or projection warping, just get a rough u/v on there that is recognizable and generally light on warping

(don't/do)Texture the high poly if you have a model that is 1. complex, 2. has a hi-poly version, and 3. is being a pain in the ass forcing you to be constantly redoing the u/v after you've already textured it, then you can texture the hi-poly with a hi-res badly-optimized u/v and then bake that onto the constantly changing low-poly, better optimized u/v. some people always work like this, personnally i avoid u/v'ing the hi poly except in special cases like this. know when to do which

but I am also not happy with the speed of my workflow

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keyboard shortcuts and WINDOWS My Computer favorites, the ultimate time saver. I don't work with udk but getting my shortcuts down and windows favorites and softimage favorites and everything setup nice meant that my time for importing a model iteration into source went from a painful couple of minutes to effortless seconds of keyboard slapping

the same is very true for the actual asset creation. create photoshop actions for pasting in and setting the blending mode on your u/v shell stamp, adjustment layer presets can be used to ensure consistency accross assets in a given 'pack', i dont use maya but little things in softimage exist like middle mouse button click on menus and im sure maya has tons of that kind of stuff to

but ya, keyboard shortcuts, customizing your tool layout, presets, maybe thats the obvious stuff but when you figure out the quicker way of doing something specifically check if its worth changing your workspace or habits to fit that new technique better.

organization! organize your actual windows folders better. what you say they are organized? thats why I said better. windows itself has simple but life changing features like i said the my computer favorites pane on the left side (which you can add to), or in windows 7 pinning specific program functions to the taskbar or start menu.

maybe this is all the obvious stuff. it all comes down to modularizing your workflow. so you've got the modeling step, many programs let you do the same thing from many different menus or shortcuts, which is the closest one? what about your viewports? I know I rarely use two of the orthoganol views so whenever I start a new file the first thing I do is get rid of them. then the export step, that should only be keyboard slapping after the first export of a given model. (remember ctrl+tab, alt+tab, and start+tab) then the check in game step is simply alt+tab + reload map/model cache/whatever shortcut.

many programs have multiple save dialogs. so you are in photoshop and want to export your art to a different format. do you go through each one and click save as in the menu? boring! perhaps you use ctrl+shift+s? faster...like a turtle? these things can be automated heck even the save for web dialog (ctrl+alt+shift+s) can save alot of time if you are sharing wip's

get a second monitor. oh you have one? get a third. oh you have one? get a fourth. oh you have one? where do you keep the key to your house?

tl;dr : find and learn shortcuts, set up automated actions, rearrange your workspace, get a second monitor

Those were some good tips Happy. I had no idea about the cntrl+shift+alt+s on Photochop.