You can use waterproof tape to secure the top edge or a wire stretcher or you can get a rigid liner.
The barrel should be sealed to prevent the wood from drying out. Sandy

Why don't you just buy a rigid liner for it? Are you talking about a 1/2 whiskey barrel? If so, the rigid liner would give you less problems. Sandy just gave me another idea if you're set on using a flexible liner.....get one of those black flat bungee cords and hook it together where it shows the least. Actually, any bungee cord would work, if you didn't mind the color.
Yes, your barrel will eventually dry out, but you could probably drill some holes in the metal band to keep it from falling as soon. (be careful that the screws/nails don't go all the way through and puncture the liner.
But my vote is with the rigid liner. They last forever.

I agree with Catherine that the rigid liner is your best choice although a really big trash bag will work. It won't be as nice looking and will need to be replaced every year.
Those 1/2 barrels are pretty rugged and if you have one of the better quality ones to start with they can last for years. A real wine barrel was made to hold liquid and so long as it never dried out completely the staves remained tight. That's why it needs to be sealed with something like Thompson's Water Seal ever so often, not to keep moisture out, but to keep it in. There are a lot of copies out there that vary greatly in quality. After all, they aren't ever going to hold wine or whiskey and they can dry out quickly and shrink. Some of them even have the screws Catherine mentioned to begin with. Even then the design of the barrel shape will hold it together. It is really hard to damage one enough to make it useless. Sandy

Yes, I'm talking about a whiskey/wine barrel. I've been looking forward to doing this for the last few years and now it seems like it isn't coming together. The barrels I had held water up to the very top rim then, because of stupidity on my part, were left empty for a week. Now the water in just the barrels stops about 6 inches from the top. I'd like to think they would "plump" up again but I don't know. I was going to put towels around the edges to see if it would help them "wick" up the water.

I think I am suffering from information overload. Some people say rigid liners are good and others say they crack. Some say to put the flexible pond liner in the wine barrel and others say to use it as it is.

I did one yesterday, no liner, but because water won't come up to the top the two tall planting containers I used for plants that can't have the crown wet, show and look horrible.

I don't understand how drilling holes in the metal band will keep them from falling apart. How do you do that and not put a hole in the barrel?

I would get the rigid liner. In the past when I've tried to use a flexible type liner in barrels they would wick water out and I would lose a staggering amount every day.

If you have something large enough to submerse the barrel in you can soak them in water for about a week and they will hold water again without liners. I do that with mine every spring, I completely submerse them in a stock tank full of water. Not exactly practical for everyone but it does work.:)

susan,
I understand what you mean about information overload. Its easy to get right at the beginning of learning about all this stuff. Don't be discouraged though.
I've heard its not too good to just use a whiskey barrel without anything because it can leach out tanins from the barrel and might hurt plants/fish.
I really think a liner is the way to go. I've never had mine crack.
What I meant about drilling holes was to put new screws into the band/wood to keep them from falling down. You'd have to use a short screw, so it wouldn't go completely through the wood and possibly tear a liner.
If you decide to go with a rigid liner, be sure to measure the depth of your barrel. Some of them are shorter than others. The one got fit the barrel perfectly to the bottom.
Good luck, and just have fun with the process. Many of us have made tons of mistakes along the way. Most of us are always changing what we have. You'll slowly learn stuff, and things will work out.

Thanks all for the support, ideas and encouragment. My husband really wants me to see if I can make them hold water on their own again so right now I have topped with towels that drape into the water and wick up to the top rim and hopefully will help them soak up water again.

If that doesn't work in about a week, I'm off for a rigid liner. Now for another question but I'll put it in a different post. :)

Susan: I agree with your husband. I have never tried towels - that might really work well. I just fill them up as far as they hold water - then just add a little more each day - try to even get a little overspray on the dry area when filling - it will take a couple weeks - you can put the plants and fish in anytime.

The liners never look good - and like you say - the barrel will dry out and fall apart.

... If you told an old barrel cooper that you were going to put a liner into his barrel to make it hold water ...

keep at it - it will hold water
Jeff

Thanks to everyone for all the feedback. I actually tried my idea with towels and it worked great. Both barrels are now up to the top of the little cut-out...as much water as they will hold.

I folded a couple of towels into long skinny pieces and placed them over the top edge. I drenched the towels and filled the water up as far as it would go. It about 3 days but everything works great now!