First and foremost Sunday newspaper. Even if you don't read it (which I don't) find the best deal (there are usually discounts for new customers) and subscribe to at least one. And when there are big coupon inserts buy one (or more depending on your family size) extras. Some people subscribe to the local paper (like Oakland Press) and the one with wider distrubution (Detroit News). If you chose just one chose the one with the wider distribution as it will have more coupons in general.
Printable coupons - there are many many sites with printable coupons, one is in my side bar so be sure to check weekly and look for updates.
Blinkies, Tear Pads, hang tags, & Peelies - Bonus coupons you can't count on but are great when they are there. Coupons that are on the package or near the item on the shelf.
Manufacturer websites - Another great way especially for organic, natural and hard to get coupons. Go to the product's manufacturer website, subscribe to their "club" and look for printable coupons there. As mentioned on the post about freebies consider sending these to a email designated exclusively for deals so as not to clog up your personal email address. Sometimes you can also subscribe to get coupons through the mail.
A note: Almost all printable coupons have a limit of two per IP address. Be sure to hit your back button (refresh if necessary) and print the limit of two. It is illegal to photocopy coupons.
Behind the Counter Coupons - Some stores like Walgreens, Joann's, Meijer's and others have their in-store small magazines with in-store coupons. So look for them or ask the service desk if they are available.
All You Magazine - Tons of coupons in this magazine. Available only at Wal-Mart or by subscription I subscribe to this and it is well worth it. Other magazines that you already purchase or subscribe to probably have a coupon or two in them as well so be on the look out.
Inside packaging - My fruit cups I buy frequently have coupons inside, also my contact solution and so do other products so check before you toss them.
Trading Coupons - start a group with your friends or look for on-line (free!) trading sites. For instance I have no pets or children so I clip all coupons send them to a trading site then order ones I do use.
A few select stores such as Kroger now have completely electronic coupons that download to your customer loyalty card. Many of these companies can be found on the Kroger website with instructions as to how to use them.
Another special note as that there are two types of printed coupons store coupons and manufacuturer coupons. Store coupons are specific to that store (Walgreens, Target, Meijers) and Manufacturer coupons can be used any where. Most stores allow you to use a store coupon AND a manufacturer coupon (commonly called stacking) to give you even more savings. Check the service desk or the coupon policy of each store to verify.