A few items I have added to my bob would not do well getting wet. I considered Ziplock bags but they are a bit too fragile to stand up to living inside my bob. What do most use to keep stuff dry?
Dry bags used for kyaking and canoeing. They work well. I bought some smallish ones to carry the kids' apples around. (the tupperware we use doesnt seal perfectly so juice from the slices would leak)
Dry boxes. You can find them in the camping section now, and are designed in various sizes to put really sensitive electronics in.
When I was in the military, I used to put my stuff in garbage bags. If I were doing it today and didn't want to use ziplok freezer bags (heavier plastic), I'd probably use hi-mil lawn bags. Your pack is sufficient to keep a garbage bag hole-free while you're carrying it; duct tape usually suffices to repair small holes. A kitchen trash bag was usually sufficient to keep my clothing dry for the two weeks of Annual Training.
If you're trying to keep small electronics dry; you can usually find "dry bags" of various sizes in the kayak section at Dick's Sporting Goods or REI, or even (if you want to overpay) at Eddie Bauer or North Face.
Conventional methods are as stated above - ziploks, garbage bags, dry bags
I've always found using my poncho to cover the bag works really well too. The outside may get a bit moist but things inside stay dry. If you have stuff that has to stay completely dry make sure it's surrounded by all your other stuff too. It's rare that a bag and contents gets soaked all the way through.
Doubling up on some ziploc bags is a good option for some items. Like a quart bag inside a gallon. Then that way you can put a couple smaller inside of the doubled up bag. Also, can go with some sealed pelican boxes if you want to take care of electronic things. But those can be a bit pricey and add more bulk to your bag. If you want an in between there are zippered plastic water proof PVC pouches you can use. Lots of options on those, do a search on Amazon.
Dry bags are the best options but you can duct tape three sides of a zip lock bag ( leave the top open ) and make them somewhat more durable on the cheap .
Dry bags, Mylar bags you can deal yourself with heat. I also like to reuse MRE bags and seal them. When your ready to use the stuff just open them. But a good dry bag is a must
If I want to pack this stuff away inside a pack, pouch or pocket I use Aloksak bags. If you think a ziplock is the right size or weight, just upgrade to Aloksaks.
Larger stuff I use sealine bags.
Lots of pelican cases and mini cases too. They add weight but are tough so good for boats and vehicles.