Walgreens Basics

Walgreens, unfortunately, has been getting more and more coupon-unfriendly lately. I would highly recommend printing out a copy of their coupon policy (link at the bottom of this page) and bringing it with you to the store. I keep one in the same pouch as my coupons. If a cashier gives you a hard time, you can always point to their own store policy. 
The bulleted items below come straight from Walgreens' coupon policy. Comments in italics are mine. 
Store Sales: Run Sunday-Saturday. Advertised in their weekly flyers, available in Sunday papers, in-store, and at http://walgreens.shoplocal.com/walgreens/default.aspx. Some sales run monthly, but they are only advertised in a single weekly flyer. I will try to note these when I can. 

Store Coupons: 

Weekly: Can be found in the flyers. They state “Good at Walgreens,” and usually have a quantity limit. (That’s per purchase – if you want more, just grab another flyer and do another transaction.) 

Monthly: Instant Value Coupons (IVC) in the Walgreens Savings Book, a pamphlet-sized book usually kept up front with the other flyers. IVCs are also available to print at http://print.coupons.com/alink.asp?go=13473cs2710 

Manufacturer Coupons: 

• Walgreens will accept manufacturer coupons for an item that is on sale. 
• The coupon amount must be reduced if it exceeds the value of the item after other discounts or coupons are applied. For example, a $5.00 coupon for a $4.99 item will result in a $4.99 coupon value. 
• When items are featured in a Buy One, Get One Free promotion, up to two coupons can be used against the items being purchased, as long as the net price does not go below zero for the items being purchased. (This is the one cashiers will hassle you about the most. They will insist that you cannot use a coupon on an item you are getting for free. Be ready with a copy of the coupon policy to show them that you can!)
• Walgreens accepts valid internet/print at home coupons

Rewards: Register Rewards (RR) which print separately from your receipt at checkout when you buy featured items. They can be used like cash (with some restrictions) on your next purchase. You do not need a loyalty card to earn Register Rewards.
The fine print:

• There is a limit of one Register Rewards (RR) printed per offer per customer per transaction. (However, because they are not tracked through loyalty cards, you can do another transaction and get another RR for the same offer. Note that you can get multiple RRs from one transaction, as long as they are all for different offers.)
• Customers redeeming a Register Rewards against the same offer may not receive another RR. (So if you do go back the next day, you cannot use the RR you earned before to pay for the same item and get another RR. You can, however, use a RR earned from a different offer to pay and still get the new RR.)
• Refer to Register Rewards coupon for expiration date. (Careful! These expire fast – sometimes as little as a week.)
• The RR coupon value cannot exceed the total purchase amount. No cash back and no cash value for RR coupon. 
• The number of manufacturer coupons, including RR manufacturer coupons, must not exceed the number of items in the transaction. (Meaning you cannot buy one item, use a manufacturers coupon for it, AND pay with a RR. You would have to buy another item, like a pack of gum.)

In short, although Walgreen seems the coolest at first for not requiring a loyalty card, they make up for it with all sorts of crazy restrictions. 

Coupon policy: http://www.walgreens.com/topic/help/generalhelp/coupon_policy_main.jsp?_requestid=37856