Breathe on your finger tips (not blow; breathe. like breathing on a cold window to fog it up.) before using them to pull at the plastic bag. Slightly moistening your fingertips provides greater friction and therefore a better chance of success.
You can also attempt to dig in to the bag with your fingernails if they are sufficiently sharp.
What often works for me is:
grab each "side" of the knot between thumb and index finger of respective hands.
"Work" the knot by twisting it back and forward repeatedly. i.e. counter rotate hands first one way then the other.
After some time it will loosen and you can then grab a loop of the knot and pull it free.
Same answer, different version - stick the tip of your tongue out, touch it between index finger and thumb, then separate the bags. Works every time, so long as you're not too worried about what bacteria or virus particles might be present on your fingers at the time... I don't do this any more, I always re-use the more expensive, bag for life, plastic bags, ...
This might not help for cycling, but when I walk with lots of bags of groceries, I put the handles through a carabiner. It's thick enough that it won't dig into your hand as much as plastic or cloth bags.
I'm surprised no one mentioned this. I know this is not a direct answer to your question, but another answer is to take your own bags! I know, I know, but hear me out - if the issue is simply that you forget to take your bags (which is commonly my issue), then here's a fix for that: have a bunch of bags always in your car (if you own one), and stick a piece ...
If I'm in the produce section, I touch a piece of wet produce to moisten my finger tips and open the produce bag.
If I'm checking out, I either touch some moist produce in my cart, or hold a small section of the bag opening area taunt and wiggle it back and forth until that section becomes separated. I then blow on it; creating a small opening to work with....
I actually used to work in a supermarket where I would face this problem almost every single day and there were a few different methods that I and other people used to help separate the darned bags:
The old classic - lick the tip of your fingers which gives them more grip on the bag when you swipe you finger across it hoping for purchase on the material, it ...
The classic way is to moisten the your index and thumb and then do the rubbing methos, usually with saliva, but of course there's germs because who knows what you've touched while picking up groceries.
Another way to get some quick moisture on your finger that does not include your mouth: If you happened to have picked up some items from the refrigerated/...
One Method I saw recently was to have a small try (could be a jar lid or a gum tin) with a wet paper towel inside. You merely have to dab at it with you finger tips and you are good to open the bags. In a super market this would be a much better option then licking you dirty fingers.
If you are not afraid to pull a little piece of a rubber glove out of your pocket or purse, this works very well. Place the small piece between your fingers and rub on both sides of the bag and the bag immediately has an opening. Very easy and works every time. I also use this method to get a better "grip" to open jars--no slip grip!
A useful hack: Protect your fingers
So, the worst thing when walking is that the plastic straps can start cutting into your hands incredibly painfully. Normally this is not too bad, but sometimes it can really become too uncomfortable. In those rare cases I have wrapped my wallet through the plastic hole of the bag(s), thus giving a nice protected area. If ...
The most commonly suggest option would be to use a carabiner as @mkingsbu stated in his answer. This is great as an option, but it tends to be unevenly allocated as far as weight distribution goes.
Your best bet is to go with something cylindrical. As you can see in the image below, it essentially takes the stress points out of the carrying/transportation ...
I struggle with this, and will not lick my fingers, etc.
I bought some Silicone Key Wraps, and put an extra one on my key chain.
I just rub that silicone ring down on the bag, and POOF, it separates perfectly.
You could use one of those Silicone Wrist bands as well. It tends to grab/stick to the plastic easily enough.
Quick solution which is more hygienic than licking your finger: touch your tongue to a spot near the cut edge of the bag. I figure that the bag, having just come off the roll or the stack, must be cleaner than your finger is in a grocery store environment.
Hold the bag to your lips where you want to open.. then take a breath & blow hard .. should separate bag enough to open fully ... of course the manufacturer could make one edge oversized or one layer plastic at the preformation .. but would not bet on it 😀too expensive for grocer or customer ..
I could never bring myself to lick my fingers. Just me I guess.
How about grasping the bottom of your shirt or blouse and rubbing the top edges of the plastic bag with it. That might give enough friction. I haven't tried it yet.
I don't want to use saliva anyway, because I think of the clerk having to touch the bag that I put spit on.
Look at the edges of the bag. Don't mess with the end of the bag where the opening is. The edges are often folded in an accordion-like way. If you put the edge between your fingers and rub, you can often get the accordion folds to open up. Once you get a little bit of an opening, you can often pull the folds apart which opens up the whole bag. Then, the end ...
Take the bag with your both hands and blow at the place where you want to open it. The air will penetrate inside the bag and will open it. Downside of this method is that you need to catch the bag with two hands, and sometimes when blowing you may produce strange squeaking sound.