I've cleared some plant growth from a few of the existing vents and will open up the crawlspace access door on warm breezy days until the space is dry. Then I'll lay, tape and secure a plastic barrier. (Don't think there's much of a humidity problem here in the SF bay area in summer.) And of course I'll install drainage outside.

I was considering insulating under the floor space between joists just as a means to improve winter-time comfort. Sounds like it might be helpful for preventing moisture transfer as well.(?)

Solving any water intrusion problems is the number one thing to do.

If you have a musty smell then cleaning the joist, sub-floor and walls will be necessary. Use a good disenfectant such as NAS 12. (Can purchace on line, it is also non toxic)

If you live in an area of California that has low humidity then venting the crawlspace is a good idea. A vapor barrier is a must. 6-mil poly plastic is the minimum thickness you should use. Make sure that you secure it at the walls with spikes (large fender washers will hold the plastic) or bricks, tape the seams. You will need to check the current codes, most codes require a vented crawlspace with a vapor barrier to have 1 sq foot of vent opening per each 150 sq ft of space. I find that a power vent(s) attached to a humidistat is almost a necessity to provide for proper air flow. Insulating the floor joist may also be necessary.