Let's start off with the stuff that's actually of interest, shall we? Most of you probably know DSfix[blog.metaclassofnil.com], and while it doesn't do anything to online gameplay itself, I strongly recommend using it with DSCfix[darksouls.nexusmods.com] which, if set up correctly, will make sure you'll find people on your GFWL friendlist faster and pretty much 100% reliably ingame (or their summon signs, rather).

DSCfix is the ONLY(!) solution that actually seemed to make things work like a charm.

Adding someone to your friendlist and reviewing them as "preferred player" might help, but it's not enough without DSCfix. You'll still find yourself getting "summoning failed" error messages, waiting for your friend's sign to appear (while getting invaded constantly) or, most likely, both.
With DSCfix, however, you'll see your friend's sign appear within a matter of seconds (usually much less than 20 seconds).

Please refer to the instructions provided there when trying to install it. I did in no way contribute to DSfix or DSCfix, so I'm neither intending to take credit nor responsibility for other people's work.
While nobody can rule it out for sure, I have yet to hear about someone getting banned for using either of those fixes. As usual, use it at your own risk (no matter how many people say it's fine).

Also, note the host still has to be in human form, the area boss must be undefeated and you can still get invaded by people who are not on your friendlist. Likewise, you should still be able to summon non-friends. Definitely make sure that both of you (host and client) use DSCfix.

A few things, while not being failsafe standalone solutions, seem to have at least a little effect on summoning, or at least aren't counterproductive. You won't need them with DSCfix though.

- Joining Way of White/Princess Guard/Warriors of Sunlight
- Adding each other as friends on GFWL
- Reviewing each other as preferred players on GFWL
- Waiting (duh)

Note that restarting your game does clear your current IP pool (quitting to title screen does not), but it doesn't seem to do much other than frustrating you.

A few things used to be advices in the past, but they actually decrease your chances to find each other. Most notably:

- Removing/Replacing summon signs, especially doing so repeatedly
- Host also spamming summon signs (might lead to the host getting summoned elsewhere)
- Client also being in human form (might lead to invasions, cancelling your summon sign)

The following, despite some rumours going arund, doesn't do anything at all.

- Using VPN (e.g. Hamachi, Tunngle)
- Resting at bonfires
- Quitting to title screen and re-loading

For most of you this will be old news, but for the sake of finding everything in one place, here's the normal summoning ruleset as it is, to my knowledge, not altered by DSCfix whatsoever.

- Host must be human
- Area boss must be undefeated in the host's world
- Be within each other's level range. A good calculator for this can be found here[mpql.net].

Last but not least, I'd like to say a few words about the concept of co-op in Dark Souls. While I understand the developers may have intended to avoid the possibility to summon someone specific (as in, a friend), I don't believe that's a good approach.
Co-op is very fun, and it is a lot more fun when you do it with friends.

While everything in the game can be done solo just fine, a few boss fights are obviously (this is not debatable, not an opinion, just sheer facts) designed for multiple players - most notably Ornstein and Smough (painful memories). The very best players on YouTube, one of them clearing NG7+ on a SL1 character, agree that this is true (and merely do it solo for the challenge, albeit it being an unfair one compared to fights such as Artorias or Four Kings).

Also, to me co-op is not something you do when you are not "skilled" enough to solo content. It's something you do because you want to have fun with your friends in a game you both like to play.

Considering all that, have fun engaging in jolly cooperation!

OH, BY THE WAY: DSCfix should also help you with finding red signs, dragon remnants or eyes of death placed by your friends, making it a lot easier to duel them too. Also, I expect it will most likely do the same for invasions via Red/Blue Eye Orb or Cat/Darkmoon Covenant Ring.