The obvious general stuff first:
Sure, what I did was extend the olive branch first. Most guilds on Veeshan hated one anothers guts, and had a history of racing for spawns, but it was pretty obvious if we all kept that up nothing would get done. It probably make it easier to work out a deal when it was noted it was in all of our best interests.
It might be a little bit harder for a european guild, since you can't just say, 'We'll all end up fighting for these mobs for 2-3 flags otherwise' as easily. Though you could always point out that, well, you raid first, and you WILL kill mobs like Bertox, Mithaniel Marr, etc for SINGLE flags if you have to, and you'd rather not be monopolizers if you don't have to.
Here's what you do- contact all the other guild leaders. Let them know you'll be flagging on what early, and find out if their early birds can log people in for flags they really need. Offer them flags. Be the nice guy and do the first step, in my opinion. Likewise, share info like mad among people you are sure you can trust. Log in your euros late for US raids, get them on their raids after you've given flags. Just talk extensively with the other guilds' leadership, offer them flag, talk about how you don't want to have to dominate the spawns, make sure they understand it's in their best interest.
A lot just depends on the personality of other guilds' leadership. If they are totally unreasonable, well hell, you are euros, just lock up the spawns until their members replace them or bail. If they aren't, so much the better, it saves you the problem of dealing with backflagging.
Now more specifically. My biggest rule was obviously they had to have beaten the mob first, and it was implicitly understood they had to be capable of killing it on their own. Secondly, it won't always be even. Some guilds just suck at backflagging and you don't want to offer them flags if you know their guild is flat out lazy and will never, ever do it. I tended to cap any single guild at one group of flag leaches they could send on a raid. Be apologetic if you have to, say you can only afford to take one group and so on, but keep the raids under control. I didn't treat all guilds equally- friends I'd flag for any reason, no questions asked, other guilds they would have to ask for any member. Really each guild had their own arrangement, but it was all understood that generally we'd flag one another when we gotten around to it. For a specific example, after we were elemental, we flagged Fires of Heaven twinks and so on for RZ, but wouldn't for another elemental guild that had slipped and were chain-failing to RZ again. After they killed RZ again, we would. My rule of thumb was they had to have at least killed the mob once in the last month to show they weren't blowing it off completely, unless they were spotting us for other flags. Another specific example- one euro guild had fennin locked down TOTALLY. To this day we have only 3 fennin kills I believe. But since we flagged them on xegony, they flagged us on fennin, since it was increasingly obvious we could walk over fennin whenever we wanted to.
The trickiest part is flagging relationships are mostly about good will. If you ***** about it being uneven, chances are it's going to collapse. Most guilds by DEFAULT distrust one another, so at some point you'll probably have to stomach at least some annoyances to get it rolling.