With an Ultimate Magus, it’s more important to keep yourself as close as possible to a single-classed spellcaster on one side than it is to keep the two even.
Thus, you want to get all 10 levels progressing your higher-level class. That is, if you enter as a Wizard 3/Sorcerer 1, you want to exit with 13th-level Wizard spellcasting, and 8th-level Sorcerer spellcasting. You do this with Practiced Spellcaster (Complete Arcane).
This gives you more of your highest-level spell slots, which are also higher level than they would be. Plus Practiced Spellcaster is a good feat anyway.
Ultimate Magus advances only one of your classes at 1st, 4th, and 7th, specifically the one with the lower Caster Level. If a Wizard 3/Sorcerer 1 takes Practiced Spellcaster for Sorcerer, his Sorcerer Caster Level is 4, which is greater than the Wizard’s 3. Therefore, Ultimate Magus 1 advances Wizard, so that the Ultimate Magus casts as a Wizard 4, and as a Sorcerer 1 with Caster Level 5 (since he has another HD, the cap on Practiced Spellcaster is lifted).
Then at 4th, his Wizard Caster Level is 6, while his Sorcerer Caster Level is 7, so again Wizard is advanced. Practiced Spellcaster has already capped out, so the Sorcerer Caster Level remains at 7; the two classes are now even.
Finally, at 7th, both classes have Caster Level 10. Since they’re equal, you may choose – choose Wizard.
By 10th level, you cast as a 13th-level Wizard, and as a 8th-level Sorcerer (with Caster Level 12).
Otherwise, you need to improve the abilities that grant you bonus spells. Consider the Beguiler (Player's Handbook II) instead of Sorcerer: you'll use Intelligence for both classes.
Versatile Spellcaster is another feat that will make your slots more flexible.
And Pearls of Power and Rings of Wizardry can help by letting you reuse slots. Memento Magicka are like Pearls of Power for spontaneous spellcasters but they are 50% more expensive, which is overpriced.