Luggage and bags:
No bags in hot summer except one for your camera, good (safe) pockets (or comfortable belt) instead. Small city rucksack for warm winter.
Clothing/Shoes/Weather Gear: Light clothes (shorts, T-shirts, better too large :-) made of natural materials (cotton) for warm/hot days all year round.
Sweater/pullover and/or jacket for cold evenings/nights (from November till April).
Shoes? Oh, yes - you should have them :-). Sandals for city walking and/or good walking shoes for trails around (dangerous reptiles and insects possible).
Add sun glasses.
Check the 10-day forecast for Yuma:
National Weather Service
Toiletries and Medical Supplies: Good sun protection cosmetics with high UV filter - use them BEFORE you start to walk around in a sun.
Photo Equipment: Be prepared for large contrasts between sun and shadow - sun filters recommended.
Miscellaneous: Tourist frige with ice cubes and cold beverages inside :-).
Definately summer attire. Warm Year round weather. Dec-Feb can be cool. The rest of the year is warm then extremly hot.
Toiletries and Medical Supplies: Sunblock spf30 or higher
Photo Equipment: If you lost or run out, don't worry--we have Wal Mart, Target, Super K-mart for those supplies.
Camping/Beach/Outdoor Gear: About 20 min. away there is Senator Wash; great place to go camping.
Cool clothes. Light long sleeve shirts to keep the sun off, you will burn out here and quickly.
Miscellaneous: Always be sure to bring plenty of water. Of course you can get it here, but I recomment buying bottled water.
I discovered that Yuma gets cold just like everywhere else in winter, I had expected warmer weather, & hadn't factored the low humidity into it, so if it's winter when you visit, bring the woollies!
Toiletries and Medical Supplies: bucket loads of moisturiser!!!! Serious!