Even when the Brewers are bad, those games sell out because of the proximity of the cities. So, yes, order in advance.

A few links to previous visiting Miller Park questions here: link, link, link, link, link.

But you probably won’t be getting in the stadium at this point. Either way, I’d go early and tailgate just for the experience.

I have just bought my tickets for the monday game. I would have hated to not get in, when I had the chance.

i think a lot of people are going to tell you to tailgate, and yes, that is really part of the milwaukee gameday experience. but, since you’re from out of town, it’s probably going to be difficult for you to have the proper tailgating equipment. let me suggest a plan b:
(1) get to the stadium early. gates open about 2 hours beforehand. get there then;
(2) use preferred parking. it’s easier to get in and out of the stadium, i find. actually, depending on your hotel, they may have a shuttle, and that’s even easier. it may not go to the park 2 hours before gametime, though, which could be a problem for (3);
(3) go to friday’s front row, which has an entry in far left field. you might have to get there before the gates open and wait in an anticipatory line. but the payoff is that you can grab a seat right above the bullpen and watch a bullpen session and maybe get a ball afterwards. i think you have to reserve in advance if you want to sit there during the game, though (not sure how that works).
(4) if you have a child, go to the fanzone. there’s all sorts of family fun there, including statues of the sausages, some pitching and throwing exhibitions, etc…
(5) after the sausage races, the sausages come out for photos by the clubhouse store behind home plate. it’s a bit chaotic, but if you have a kid (or easily amused companion), you can get photos and even autographs of your favorite racing sausage.

i agree with a lot of the other comments in the links presented earlier. also, if you do have a kid, maybe do the sprecher’s brewery tour. it’s not a very interesting tour (they show you two rooms), but you get to have a few drinks after, and they have some delicious sodas. (my favorite is the root beer, but my gf described the ginger ale as “liquid cake”.)

you’ll have a good time. i brought a few of my new york friends to a recent game, and they all enjoyed it (when not checking their blackberries).