What is the maximum possible bonus to Karma?

I've been saving my Jugs of Liquid Karma from completing daily and monthly achievements, and I've got quite a few stashed away. Since it's possible to get bonuses to Karma through things like guild banners and some food types, I want to make sure that I get the most use out of them that I can.

What is the maximum possible Karma bonus and what do I need to do to reach it?

Solution 1:

With regards to using Jugs/Flasks/Gulps/Drops of Karma most efficiently, the largest possible bonus at this time is 95%.

To obtain this bonus, you'll need to:

• Use a Karma Booster for 50%
• Touch a "+10% Karma and +10% Experience" Banner
• Eat an ice cream food item for 5%
• Be under the effect of the "+15% Karma for 24 hrs" guild buff
• Have an item with a Karmic Infusion in it equipped (15% bonus).

Altogether, these bonuses will combine additively to give a 95% bonus.

Additional Notes:

• 95% is not a fixed cap, it is simply the largest bonus available at this time. It is possible that more Karma Modifiers will be added at a later date.
• It's worth noting that although the Outmanned buff claims to add an extra 33% to Karma gain, it does not work for liquid karma items (and as Raven Dreamer pointed out, it actually does not work at all right now). The Outmanned buff no longer claims to add a Karma bonus (Thanks for pointing this out, Brian).
• Thanks to Metemer for pointing out that there is now a 10% Karma Banner.

Source: Guild Wars 2 Wiki page on Karma Modifiers

Solution 2:

As of the 2013.02.26 update, there is a banner that adds 10% karma bonus instead of the usual 5%. It doesn't stack with the 5% therefore the new maximum is 95% making a jug give 8775 karma instead of 4500 (4275 more).