Also, here's how to avoid getting jumped by gankers (using invasion items, doesn't apply to ring-based summons):
Don't use your invasion item next to bonfires. Find different spawn locations.
It's an easy fix!
The most popular spawn locations are the ones that are closest to bonfires. This is because it's most convenient for Darkwraiths, who have to travel back to where they were to reclaim lost souls if they died during the invasion. People have caught on to these spawn locations and sometimes camp out by them.
But people who wait by one spawn location probably have no idea where the rest in the level are, simply because they aren't used as much. Sen's Fortress and Blighttown have 7 in total, Anor Londo and TotG have 6, the Painted World has 4...
Even if everyone familiarized themselves with all of the locations, that really only helps the invaders, because that's too many locations for a host to manage (with the exception of only a few areas, like Duke's, which only has two locations).
One example:
In The Painted World, invading by the bonfire spawns you at the end of the bridge at the beginning of the level. I've been jumped there on multiple occasions. And even if you don't get jumped outright, most people expect invaders to come from there, so it's likely you'll have people coming after you or waiting for you around the bonfire. It's the most popular place to invade from, but it's also easily the worst.
Invading just a couple steps past the shortcut door in the courtyard will spawn you in front of the annex door on ground level, and invading anywhere in the tower will spawn you at the top of the tower by the red sign soapstone corpse. These are both a very short walk from the bonfire, so it's not even inconvenient if you have to travel back after failed invasions. I've done a lot of invasions from the tower now, and not only have I never been jumped, but it seems that the vast majority of players expect me to come from the bridge.
I've been able to surprise players and I've had a much better rate of success in 3 on 1 fights than I've ever had otherwise, simply by putting a little more planning into where I spawn.