Channeling is when little pockets in your coffee puck get water passing through them more easily and in turn gives you an uneven extraction.
Think of it like a hose you'd put your thumb over, the part where the water comes through faster is because it has found the easiest way out. Similar to when you have gaps in your coffee bed that allow for water to pass through quicker.
Distribution - The way in which we distribute the coffee in the basket, making sure that it is nice and level. There are distribution tools that you can buy to help with this but at first I'd try to improve your technique of good distribution before digging deep in your pockets.
Pre-Infusion - This technique can not be performed on a lot of machines so I understand why this might be difficult. If your machine does provide the option to have pre-infusion at a low pressure then it will help to distribute the water more evenly through the puck.
Break Coffee Clumps - There are little pin tools that will break up the clumps of coffee you get from grinding your coffee. Breaking up the clumps will allow the water to pass through it more evenly.
Tamper - Are you using the right sized tamper for your basket? If the tamper does not sit snug in the basket you may be leaving yourself with room for the water to pass along the edges of the basket, check what you're using.
Filter Basket - If you're using a basket that has rough jagged edges at the bottom this will lead to water pouring through it unevenly, you can shop online for more precision baskets but it can get pricey.
Naked Basket - To see where your espresso might be channeling you can use a naked filter basket as seen above, I use this in my training to demonstrate to students where you find gaps in your coffee bed.
It all starts with technique, if you can better your own technique tamping and distributing you most likely won't need to spend an arm and a leg to reduce your channelling... hopefully
Have you got any other ways to reduce channeling?
Isn't espresso just as infuriating as it is beautiful?
Would you like more information on any of the topics above?
Keep at it!