One place to start is to examine the list of spells you have available. Try to decide whether each spell is something you'd always want to prepare, or if it is something you'd only occasionally want.
You should create scrolls for any spell which you wouldn't regularly prepare, but when you need it, you really need it - detect invisible and knock are a couple of good examples here. This will let you prepare more of the spells that you will routinely want. Also create scrolls for a couple of offensive spells - use these as a reserve for times when you deplete your spells and desperately need another bit of punch for a battle.
Another good use for scrolls is buying them to expand the spells in your book. Instead of buying a scroll and simply using it, buy one, add the spell to your book and then make as many scrolls of the spell as you want.
For magic items, a few of my favorites:
1. Heward's Handy Haversack. I can't say enough about the merits of having one of these. Wizards often end up with a low strength, which in turn means they can't carry too many items. There's also the extra ability of the item that you can retrieve anyting from it as a standard action, rather than a full-round (which is the norm for extra-dimensional spaces). At 2000 gp, this is a steal.
2. Rings of Wizardry. A bit pricey, but one of the best items a wizard can have. Doubling the number of spells for a particular level keeps you tossing spells out much longer than normal.
3. Metamagic Rods. Another set of items that any aspiring wizard should have. These are great for allowing you to pump up spells you're casting without having to pay the penalty of using a higher-level spell. Furthermore, you can use them as the situation arises too. The silent spell rod can be a lifesaver if you run into a silence effect.
5. Stat Boost items. Boosting intelligence is by far the most critical - more spells and higher save DCs. Con and Dex are about even for the next most important.
6. Pearls of Power. Being able to re-use a spell after its been cast. What more is there to say?
6. Bracers of Armor. This one is kind of a tossup in my mind. If you can get a bonus of +4 or better, go for these, otherwise rely on mage armor.
7. A mithral buckler. This one often gets overlooked by many wizards. A mithral buckler has no armor check penalty or arcane spell failure penalty. A wizard can use one pretty freely, and it can be enchanted for a bigger AC bonus. The penalties for not being proficient with it are negligible for a wizard as well. And it stacks with Bracers of Armor/Mage Armor too.
8. Resistance Items. A cloak of resistance to up your saves is handy, as many of the true threats you'll face require Fortitude or Reflex saves, neither of which is a wizard's strong suit.
9. Wands. I actually put these fairly low on my list. A wizard ideally should be relying primarily on his/her spells. If you're not, you may want to re-examine how quickly you burn through your spells and whether the way you're typically using them is effective or not.
One thing you're going to find with magic items is that the more effective a spell is in combat, the more costly it will be to generate the effect.
I also wouldn't discard custom spells out-of-hand. You really need to take a look at the style of play the rest of your group (including you) has. Try to come up with new spell concepts that complement the strengths of the group and help protect against weaknesses. Look in particular for minor effects that stack well with or enhance other abilities.
As an example of one I had previously created: one of the members of the party I was playing in specialized in tripping (yes, I can hear a groan coming from many of you). I created a first level spell called stability which made the target gain the dwarf's stability ability for a time. It wasn't powerful offensively, but it considerably enhanced the tripper's ability to avoid being counter-tripped when his trip attempts failed, thereby enhancing the effectiveness of the group as a whole.
As far as stacking spells together for greater effects... I highly recommend you work with whomever is the divine caster of your group. There are a number of spells from the arcane/divine spell lists which provide similar bonuses, but stack well together. A couple of lower-level spells combined together can often provide a benefit superior to that of a higher-level spell.
Consider Mage Armor, Shield, Barkskin, and Shield of Faith. All provide different types of AC bonuses, so they will all stack (and yes, I realize shield is a personal spell, but the concept is there). The improvement isn't as linear as you may think. I would argue that +12 to AC is often more than 3x as effective in combat as a +4 to AC.
Different bonuses that stack can also be found in in attack rolls and damage rolls. Never forget that size is a sometimes an overlooked way to enhance damage too. Bull's Strength + Enlarge Person + Magic Weapon + Aid + Divine Favor can all stack stack together (the latter two being divine only). A single friendly character with all of these magics running can be quite a bit more formidable than usual. These are all 1st and 2nd level spells, so if you have the time to pre-cast a spell or two before a combat, do so!
