Extra Summons at 1st, 5th, 11th and 15th levels. As a master summoner you want as much of summon monsters per day as you can.
EDIT: I assume that actual availability pattern for Extra Summons is the same as for Extra Evolution feat/Improved Natural Armor evolution as they both used the same language as Extra Summons before clarifications for them were issued (the fact that they can be taken once per five summoner levels contradicts prerequisite of one level of summoner). Check with your GM if he agrees with me on that interpretation.
Superior summoning at 3rd level.
At seventh level? Ok, this is the first time you have to pick right... If you are making a human/half-elf master summoner that focuses on demons *and* plan to play to very high levels, I'd say get a Skill Focus (Planes) as a bonus human/half-elf feat and take Eldritch Heritage (Abyssal) at 7th, Improved Eldritch Heritage at 13th and Greater Eldritch Heritage at 17th. This is a very long term investment, however, albeit the one that allows you to summon 3 glabrezu/nalfeshnee at 17th level with single casting.
This leaves us with 9th and 19th level feat choices. Improved Initiative is always a good choice, especially for summoner who conjures his minions with a standard action and they act in the same turn.
D&D 3.5
Beckon the Frozen (Frostburn, p. 47): summon creatures +1d6 cold damage
Augment Elemental (Magic of Eberron): attack and damage bonus for summoned elementals
Pathfinger:
Take feats that keep you alive ... that's it. The summons are strong engouh but if you fall it does not matter anyway.
Toughtness
Dodge
Breiti