Downtimes that say "...and I tell all this to the mage who always wears a purple robe" are a pain in the neck. Much better is "...and I'm telling all this to [charactername] - I've already contacted them and they've agreed the conversation" with a matching bit in [charactername]'s downtime that says "[thingy] told me all this cool stuff." This saves the refs having to reply "It is up to you to do downtime contact with other characters (barring sneaky stuff), do you need [charactername]'s email address?" and then having to ask people whether they're okay with giving out email addresses... Faff.
Even worse is when a downtime mentions doing something to/with/about someone with only a vague description (or possibly name) that none of the refs recognise. Refs often forget what they wore for an NPC, or even what they named them - and this goes double for non-ref monsters. If I send three villagers into the bar it is very unlikely I'll ever know what they were called unless a PC mentions it in downtime. Therefore naming which physrep you are referring to (as well as name if known and a description) is a good idea.
Following on from above, are the downtimes where someone wants to do something with another PC, but there's no mention of this in the other person's downtime. We need an email from both of you. The best way to do this is for one of you to compose the "what we do" email, and CC it to the other person, who then replies all "Yes, I do this." - if either PC is doing other stuff separately that can go in a different email. This way the refs can be absolutely sure that both players agree to the action.
Something that is a fairly recent niggle is characters whose names I don't know. I'm awful at names, but this year I made a real effort to know all the players' names and their characters'. Then people start making PCs with Disguise who go by so many different names I can't keep track. If you're using disguise and have different identities/names don't assume the refs will remember what name you're going by - mention in every downtime what you're doing under which identity, but make it clear which one is the real one.
The reason these things annoy me, the really horrible bit, is that they're not too difficult to sort and getting them muddled up can really screw up what would otherwise be a fantastic bit of PC-plotting. These are things that only make the refs' lives harder if they frantically try to sort them out in order that the cool stuff can go ahead. Unfortunately the more time it takes to work out what's actually going on, the less time there is to write up what happens when you and your mates go to Windy Peak and tell the air elementals that the College of Earth is planning to dispel them all.
In general a downtime plan is composed of several things:
It is entirely acceptable to email the refs with a downtime that can be summarised as "My character wants to do X, so far as I'm aware I have resources A, B and C. I spend the week talking to people I trust/reading books/praying (season to taste) to try to come up with a way I could achieve X. This is generally for when you really really don't know where to start - the refs will in general try to give you a clearer picture of your resources (we can often point out ones that people didn't realise they had OOC but that their character would have been aware of), and to suggest some intermediate goals.
Similarly, if you have come up with a cool multi-step plan it's usually a good idea to tell the refs. This allows us to warn you if you've misunderstood something (nothing worse than getting to "and then I use X, Y and Z that I have already prepared to create MING!" and us having to say "erm... sorry... you should have known that MING needs Q not Y." The other thing it allows the refs to do is to run around filling in the bit of world you're about to step into - cunning plans usually go "off the map" in some fashion, and this requires decisions about what exactly is here. The longer the refs have to think about it the better.
Reporting. Now, let's say you've just been on a linear and wish to report back to your temple/college/minions. The best way to do this is not "I tell them what happened on the linear." Please bear in mind that we have no idea what your character thought was important enough to inform them about. Of course, in most instances we can work out what you mean, but it's very easy for us to miss what your character thought was the most important point. I'm not asking for an IC written report (although those are cool) - simply mentioning the three points that your character thought most pertinant (and one thing that they left out, if anything) is orders of magnitude better. The other thing you should mention is what slant you're putting on it - purely informative, self-promoting, dropping someone in it...