One of my old guilds long ago had a probationary period for all new members that joined. Basically once a new member joined they were informed they had about 2-4 weeks (can't remember just how long) to be sponsored by two senior members of the guild.
What this did was encourage new people to group with others in the guild, which in turn showed the guild what type of person they were by interacting with them. Most full members would take time to run groups with potential new members to get to know them, and see if they were worthy of sponsorship.
The end result was a guild filled with members that were all decent, and helpful. Of course this was only so because if an A-hole did make it into the guild, the leader would kick them after one warning (and there was no coming back).
There were also a few other rules they enforced, like they looked down upon someone that typed as if they had brain damage, spammed, or spaz'd out in a cussing frenzy.
I have seen good people driven away from guilds/linkshells many times from a few causes. The worst one would be lack of leadership, people need to know the leader is pro-active. The other would be morons that made it into the guild, and either openly spammed crap into the shell, or just annoying in general and were never booted.
Either way, good luck with your guild, and remember to smite any morons that infiltrate your ranks.
Survivor of the Twinkling Treant - 2003
FFXI - Cerberus - 99 PUP, 75 BST-RDM-BLU
FFXIV - Besaid -> Balmung - 9/23/10