I'm making my first trip in August. Planning to put my stuff in my backpack (that I uses for backpacking). I have some individual waterproof bags. But, I'm also thinking about using a trash compacter bags as a liner. I use this when backpacking, but don't have to worry much about tying it at the top.

I was wondering if there is a "best" way to tie the bag when using it as a liner on a canoe trip.

I'm really excited about my first trip....4 of of heading out for 6 days in early August.

PhantomJug. Thanks.  As for name...you would think so....but that's another story.

 Some of us use a double bag system. IF you have a backpack that has a separate bottom area for sleeping pad that you can unzip to make ONE big bag, this method serves you well.
 People use heavy duty garbage bags, or makeshift liners but for a few bucks, I would get two of these. Either way you go, get as close to 6 Mil as you can.  
One inside the other, outer bag is your WEAR bag. I fold them them down by gathering them together as close to load as I can, then twisting them 180 degrees, and tucking the excess into the front or back of the load. I repeat this with the outer bag, tuck opposite of the first one. Don't worry about the length, get long one's, you can cut them down if need be.
 I have 4 trips on same two bags, although I had too duct tape a few rips in the outer bag from abrasion by the pack body.
 I agree about external frame pack being unwieldy for tripping, but I've seen in done and IF you have an internal frame pack, you should be fine

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I just twist the top of the plastic bag, double it over, and wrap a large thick rubber band around it.

I just roll it up. Cinching the pack flap & straps will keep a tight seal.