+1 to everything kcpup said
What we are doing here, homebrewing, isn't an exact science but if you want to produce the best beer you possibly can the three biggest things you can do for yourself are:
1) Do your due diligence to make sure your sanitation is up to snuff. Clean and sanitize the hell out of everything, twice if you need to. Replace your soft plastics like hoses often.
2) Get yourself good fermentation control. sitting the carboy in the cool closet with a wet t shirt and fan won't do in this day and age.
3) Pitch appropriate amounts of healthy yeast.
If you can get a grip on these three things, you've capture the ability to make not just good beer but likely award winning beer.
Oh and by the way. I kept some extra wort from a Hefe I did last summer and used it to prime a few weeks later, it seemed fine. I knocked it out into some sanitized mason jars before cooling my wort, capped the mason jars and cooled them quickly. I don't see the problem with keeping some wort around for a while as long as you've treated it right. I'd try to snag some before my first hop addition but you want to let it boil for a little while too. I'm sure you could manage this with a small change in process and planning. DME is just a really cheap and easy way to make starters.