Calculating Spell DC
10+Spell Level+Relevant Ability Modifier before relevant feats and class abilities.
Calculating Caster Level Checks
To make a caster level check, roll 1d20 and add your caster level (in the relevant class). If the result equals or exceeds the DC (or the spell resistance, in the case of caster level checks made for spell resistance), the check succeeds.
Feats to Increase your DC
Spellcasting Prodigy: Gives +1 DC
Spell Focus: Another +1 DC
Greater Spell Focus: Another +1 DC
Shadow Weave Magic: Another +1 DC.
Skill Focus Spellcraft Bonus from Master Specialist
Increase your effective Caster Level
Orange Ioun Stone: +1 Caster Level
Create Magic Tattoo (Spell, Spell Compendium): +1 Caster level
Ring of arcane might : +2
Prayer bead: +1
Practiced Spell Caster +4 caster level
Spell Penetration: +2 Caster Level (caster level checks only)
Spell Penetration, Greater: [stacks with spell penetration]