So there are a lot of options for life gain that can work out beautifully for you, BUT, the question is, how much do you want to deviate from your current human-based format? Further, how much were you looking to spend to improve it? For instance, Serra Ascendant is a NICE offensive weapon for a life gain deck, but they come at the cost of almost $4 each.
Some cards to keep your eyes open for include:
Celestial Mantle - it's legal in modern, pumps up your creatures, and lets you DOUBLE your life total if you manage to score a hit with that creature. There are plenty of combos you could run with it to make a creature unblockable if you splash some blue into the deck. Otherwise, think flyers (like the above mentioned Serra Ascendant).
Martyr of Sands - it's legal in modern, is a one drop, and can easily pump your life total if you're running mono-white. Think, first turn you drop it and a land, so you have 5-6 cards in hand. Second turn you draw another card, up to 6-7, assuming you have 2 lands, you can still reveal 4-5 cards and gain 12-15 life.
Elixir of Immortality - I don't really think this needs much explanation.
Chalice of Life - this is legal in standard and modern, is relatively cheap, and when it transforms, is a MONSTER card if the game is stalled (or if you or your opponent has a huge defense).
Beacon of Immortality - pretty self explanatory here. Six mana, double your life, shuffle back into deck, rinse, repeat.
Rhox Faithmender - the pic likely won't work on this one as it's an upcoming card in M13. You can see the specs at http://tappedout.net/mtg-card/Rhox-Faithmender/ - again, doubling life gains whenever possible. If paired with the Beacon, that means multiply your life total by four.
Cradle of Vitality - again legal in modern, and this one goes under the radar from what I've gathered. However, why limit yourself to just Ajani's Pridemate when you can have this enchantment that lets you target ANY of your creatures to give them +1/+1 for EACH life you gain, meaning if you gain 10 life from a Martyr of Sands, pay two, and BAM, your creature of choice gets +10/+10.
Felidar Sovereign - who said you have to actually hit your opponent to win the game? Legal in modern, 4/6 with both vigilance and lifelink. And, y'know, as an added bonus, if you have over 40 life, you win the game in your upkeep.
I hope these suggestions help. White has definitely become the center for IMMENSE life gain. I would suggest keeping some exiling effects in the deck to stop anything that's too big to handle, but realistically, if played correctly, your life gain can outpace the damage they will do to you. One last suggestion would be to include 2x or 3x cards that give one (or more) of your creatures trample, as it sucks to have a 24/24 Ajani's Pridemate that can be easily chump-blocked by a 1/1 token. Here are some equipment that fit the bill (http://deckbox.org/games/mtg/cards?f=27
F1aXBtZW50).
Best luck with it!