Several suggestions:
1) Improve attic venting. Consider passive ridge vents and/or the addition of a power fan with a thermostatic control. Pick a quality, quiet fan. Make certain that the items that you are storing don't interfer with air flow. For example, if you have soffet vents then don't block their intended air flow with the items that you are storing. Etc.
2) Consider purchasing one or more low cost digital thermometers. Place them on stored items in the attic and periodically check the thermometers. They should have a memory which records the highest and lowest temps since the last time you reset the thermometer. This will let you know if you have adequately improved attic venting. These thermometers are quite inexpensive and very useful for many applications.
3) Don't waste your effort on adding insulation anywhere in the attic for the purpose of protecting items stored up there. The dollars are better spent improving the attic venting.
4) Remember that an unfinished attic such as yours in not engineered for storage. Of course, almost everybody uses such an attic for storage.
Distribute heavy objects somewhat uniformly in the attic. Obviously your attic is a bit over-engineered and you are not going to cause problems with some modest amount of attic storage. Still, avoid too many heavy objects such as boxes of books, paint cans, etc.
5) Protect against rodents. You don't want them chewing on your stored blankets and such. Also, you don't want mouse droppings in half of the cardboard boxes in the attic. I keep 2 baited rat traps and 2 baited mouse traps in my attic. They are located in a central part of the attic, but in a spot where I'm not likely to set them off. I glue several large sunflower seeds on the bait areas and I periodically check the traps to see if they have been set off or if the bait has been gnawed. FYI - the smell of the glue does not stop rodents from going for the bait. My favorite is super glue.
6) You will still have to be selective in what you store up there. Don't put the good holiday candles up there! Also, remember that plastic bags will break down over time. For our seasonal items, we remove the bags from the items as we are removing them from the attic and replace with new bags when the items go back into the attic. I use the super large recycling bags whenever possible - these are clear and that aides in identifying stored items. For items which are in "long term storage", I check the plastic bags every year to make certain that the plastic isn't gettting brittle.
7) Start off organized. Put scrap boarding down for walkways so that you can move around safely and avoid stepping through the ceiling below. You'll be surprised how much planking your neighbors will put out with their trash over the course of a single summer. It's free.
Label all boxes on several sides with very large lettering. Plan ahead and store like items in certain parts of the attic, with frequently needed items closes to the access hole.
Good luck, Gideon