Feats:
Spell Focus +1dc for 1 school
Greater Spell Focus +1dc for same school as spell focus
Varisian Tattoo will get u +1cl and +1dc to one school of magic
Bloatmage Intiate will get u +1cl and +1dc with a bit more downside
Trait:
Loreseeker will allow u to pick 3 spells of any level and u get +1cl and +1dc to those..
Feats:
Elemental Focus and Greater will boost a particualr energy type and additional +1 for each..
Spell Focus +1dc for 1 school
Greater Spell Focus +1dc for same school as spell focus
Varisian Tattoo will get u +1cl and +1dc to one school of magic
Bloatmage Intiate will get u +1cl and +1dc with a bit more downside
Trait:
Loreseeker will allow u to pick 3 spells of any level and u get +1cl and +1dc to those..
Feats:
Elemental Focus and Greater will boost a particualr energy type and additional +1 for each..
Neither Varisian Tattoo nor Bloatmage initiate increase your spell DCs - only your caster level.
Late down the line and a huge investment but Eldritch Heritage, Improved and Greater Eldritch Heritage: Arcane nets you another +2 to the DCs of one school.
1st: Start with a 20 INT from your base abilities + race
2nd: Spell Focus and Greater Spell Focus (Conjuration mechanically is the "best" school but pick one you like and stick to it).
3rd: Headband of Intellect, invest in one as early as possible.
If you have a minion or adventuring companion which can demoralize opponents, that -2 to their saves is basically like +2 to your DC!
My 10th level ranger's large wolf companion, with 3 intelligence, Intimidating Prowess, Dazzling Display and trained in the Perform (Intimidate) trick, at +21 intimidate vs. medium-sized opponents, almost always demoralizes foes within 30 feet... spellcasters love it :)
As David was saying, it is easier to lower someone's saves than it is to raise your own DCs. If you go crazy with the debuffs you can have some pretty high penalties to saves.
Some of the basics:
Shaken condition: -2 to saves and most other rolls
Sickened condition: -2 to saves and some other rolls
Negative Levels: -1 for each negative level
If you had both conditions and one negative level, that is a penalty of -5, which is pretty heavy.