I'm a strap junkie. And a Nite-Ize junkie.
Someone mentioned the Gear Tie, which I like: https://www.niteize.com/product/Gear-Tie-ProPack.asp
I've also used the Knot Bone Adjustable Bungee: https://www.niteize.com/product/Knot...ble-Bungee.asp
And the Flat, Adjustable Bungee: https://www.niteize.com/product/Knot...lat-Bungee.asp
I prefer the flat for securing a load, but sometimes the bungee pulls double duty as part of my tarp rigging, and in that case, I tend to prefer the other.
These bungees address some the concerns of the anti-bungee crowd. They're adjustable, so one bungee can be adjusted for multiple uses. And they have clips at the ends, rather than hooks, so there's far less chance that a bungee will break free, have that hook come swinging around and take out an eye, releasing tension on the other end, so that hook comes loose and gets the other eye, and all of the sudden you're blind and crying by the side of the road while your unsecured load falls into the ditch. Never happens with these.
But, in all seriousness, I have had an issue where a bungee came loose and dropped an end into my rear wheel, wrapping itself around the hub, leaving me standing in the rain at 1am with a flashlight trying to figure out how to get the bungee and wheel to separate into two distinct entities again. So I like the clips vs. hooks.

Non-Nite-Ize products:
I've used a cargo net to good effect. Particularly for securing shoes or wet clothing that you don't want in your pack.
Surly Junk Straps: Cargo | Parts and Accessories | Surly Bikes
The have a loop version that I haven't tried. Yet. (Re: junkie)
I use Salsa Anything Cage straps on my Anything Cage: Anything Cage Straps | Parts & Accessories | Salsa Cycles
But I also have the longer version, which has a buckle. Those have been great for securing a load to the rear rack that you want to access throughout the day. I often carry a cooler, and having the buckles available to easily remove it has been handy.
The Revelate Washboard Strap has been useful on my Anything Cages as well: https://www.revelatedesigns.com/inde...WashboardStrap
From what I understand, it's basically a ski strap. It has a little give, and is great for strapping down a load that has no give (like a bottle of fuel, water, beer, whatever). It's not terribly long, but for smaller items, it's handy.
Also a nylon belt as part of my travel clothing has come in handy as an emergency strap, something similar to this: https://www.rei.com/product/101415/b...lliptagon-belt
Just remember that nothing is holding your pants up when you get off the bike to walk around.

I wonder how much light my gear would be if I left my straps at home?