First off, when you say you're going wine tasting in California anyone who knows wine country will usually first as you, "are you going to Napa or Sonoma?" There are actually two counties that span wine country; and anyone will tell you that Napa is expensive. Sonoma has many amazing wineries, so if you're worried about price don't think you'll be missing out by not visiting Napa. Smaller tasting fees, if there even is one, and most times they waive tastings if you buy a bottle make it great for a budget. I have friends who rarely go to Napa anymore because of this. We have yet to go to Sonoma because we are making our way through the massive list of places on our list in Napa first (plus it's easier for us to get there).
So how do I plan the spots we visit? Generally, depending on time, we try to hit 2-3 a day. One in the morning, stop for lunch, then two more in the afternoon. Because we live South of Napa County and have always stayed in downtown Napa when we've visited, we usually go to the spot furthest North first then make our way South. There's two main roads in Napa (Highway 29/St. Helena Highway and Silverado Trail) that run parallel to each other. 29 gets packed with cars as the day goes on, so I also try to pick places there earlier in the day and then elsewhere later. I also always make sure there is at least one winery on or off of Silverado because the views on that drive are spectacular. Really, it's gorgeous no matter where you are, but it's less developed.
And as I said, always stop for lunch. When you go to a tasting, you usually drink between 1 1/2 - 3 glasses of wine depending on how many varieties to taste and how much the server pours for you. You will be drunk and it will happen fast. Lunch is necessary not only to help slow you down and sober you up, but to pad your stomach so you're not a mess in the afternoon. We usually eat in St. Helena because there are many wineries we like near there and there's some really cool olive oil and vinegar tasting rooms in the downtown area too.
Lastly, the time of year is also important. If you *must* see grapes on the wine, the summer is the time to go, July through early September to be exact. But be prepared to deal with crowds and sometimes it can be HOT. Sitting outside dying and drinking red wine really isn't fun. But you will see a lot of green and there's always summer wines to taste. The off season on the other hand has less traffic, cooler (and unfortunately sometimes rainy) weather, and the vines look like twigs. But you get more personalized service and better deals. So there's pros and cons to each situation.
Here's a few words about some of the places we've been to that we really like:
Casa Nuestra - We liked the wine here, but of all the places on this list it's probably at the bottom (that's not to say it's not good, we've been to about a dozen places at this point, so that's like 5 out of 12), but it's a really cool vineyard. They waive tasting fees if you buy a bottle, tastings are private, by appointment only, and are outside. There's goats and the scenery is gorgeous. Eco friendly too. A really cool place considering many other rooms are busy, high tech, and can be inattentive. (Also dog friendly.)
Trefethen - Our favorite wine ever. We'd join a wine club here if we could. I literally like everything they have. Whites, reds, it doesn't matter. It's all amazing and the main guy who runs the tasting room is really great too. When my family came to visit we took them here and they were raving about how their wine was the best out of all the places we visited. 5 out of 5 of us can't all be wrong. (Also dog friendly.)
Duckhorn - A friend's girlfriend recommended this place to us. It is EXPENSIVE, but the wines are nothing short of amazing. The tasting room is in an old victorian on a quiet country road with beautiful scenery. Against our better judgment we splurged and bought a bottle to open on our 15th anniversary. It might have been the best decision we made that trip.
Cuvaison - A French style winery, Cuvaison has the best Chardonnay ever. Really. I'm not lying. My friend Jen left with two bottles she liked it so much. I've been eying mine for awhile now but am afraid to open it because it's too good to drink on my own. The guy who did the tasting with us was super friendly and we all left extremely drunk, which I'm sure made him happy considering the amount of wine we got. (Also dog friendly.)