Couple questions regarding my first time with Avis.

1) What is Preferred Service? Do I need to qualify for it? If so, how do I do so? Will my corporate program affect this in anyway?

2) Is the USAA rate the lowest out there?

3) Can I double dip points? If not, are the Avis points more useful than sending them over to UA?

4) Anything else I should know/do?

Thanks!

Like Gary03 said, Avis does not do pts. You will have the option to recieve miles with UA however some say miles are not avaliable at off-airport locations.

It's nothing, really. When you sign up maybe your name will be on the reader board on arrival, maybe it won't. You're not a candidate for upgrades with PS but sometimes you get them at random.
Maybe, maybe not. Check the discount code thread on this forum. It's like playing Battleships. Some of the codes don't seem to do anything, others cut the price in half.
Like the other poster said, no such thing. The Avis loyalty program was upended this year anyway -- they did away with Preferred Selecr, which had no benefits I could see apart from a 50% chance of a Wall Street Journal on the dash, and brought in something called Avis First, which I was promptly thrown out of for not having enough recent rentals. So... Hertz hasn't thrown me out of #1 Gold although long periods used to pass between Hertz rentals... but now I give all my business to Hertz.
Depends on the station you're renting from. There are some newer '08 cars coming in, but in the spring I used to see a lot of dirty 25k dogs, in every vehicle class. Scrutinize your receipt VERY carefully for fake gas charges.

They don't try as harder as they used to IMHO.

Not sure that this qualifies as double dipping, but there is a new Avis First program based on number of rentals or days...

You'll get a coupon for a free weekend rental every 3 qualifying paid rentals up to 8 a year. So far, that's all I get from Avis 1st. There are alleged upgrades.

You can get miles or points in a ton of programs from Avis. But there will be taxes added for some programs in some states. Beware - some employers do not wish to pay an extra $2 a week so you can get 250 miles in your FF program.