How can I make myself more visible during the day?
Maybe something to put on my helmet (so it's easy to remember).
1. Wear hi-visibility color on your torso.
That really bright yellow/green color is best. The orange is pretty good, too.
I've seen stuff that's striped in both colors or is primarily one with a some bands of the other, and that seems like probably the best option, so that you still stand out against a background that matches the hi-viz color.
Helmet and leg stuff wouldn't hurt, but will tend to be less effective than torso, since the helmet is fairly small and your legs are fairly low.
There's a lot of available options for getting hi-viz colors on your torso. Most include reflective bits. Things like: a safety vest, a hi-viz safety triangle belt, a hi-viz cycling vest, a hi-viz cycling jacket, a hi-viz cycling jersey, a hi-viz t-shirt, or even just a regular t-shirt in a bright visible color... Most bike-shops will have a hi-viz jersey and hi-viz cycling vest or cycling jacket available. "Work wear" shops that sell uniforms and other work-clothes will often stock a variety of high-visibility stuff.
2. Attach hi-visibility colors to the bike.
A safety triangle hanging from the back of the saddle. A bag under your saddle in a hi-viz color. Hi-viz colored panniers. Hi-viz colored reflective tape on your fenders or other parts of the bike.
This will be lower and smaller than on your torso, but couldn't hurt.
3. Bright flashing lights
You need unusually bright lights for them to have a chance of being at all useful in full daylight.
I have the Cygolite HotShot that won the Tail light review on our blog. It's definitely visible in daylight when it's in a flashing mode. My 300 lumen headlight in flashing mode is also visible in daylight.
4. Ride visibly and predictably
Ride where you'll be seen. Ride where cars will be looking for you. Don't ride in the gutter or hug the curb. Merge into the lane position a car would use to go straight when approaching a place that a car can turn across your path. Claim the lane when appropriate. Take a safe cycling course, or at least read up about Vehicular Cycling.
This is probably actually the best thing to do.
5. Ride with other bicyclists.
Hard not to see a pack of 20 cyclists riding close together.
Safety can be thought of as a packet of swiss cheese slices. Each slice is an imperfect barrier (crashes progress through the holes) but if you have enough slices you can eventually block all the holes and have a strong system preventing crashes.
Visibility and a helmet is the last line of your swiss cheese defence.
I absolutely agree with riding predictable vehicular style but always be prepared to take evasive action. The more escape paths available the better you can deal with the unexpected.
People see what they expect to see. Looking is different to seeing. Look here and you will see.
Now you will hopefully never assume a driver has seen you. This does not mean you give way to everybody though, usually just a judgment on safe buffer distance and speed. Is the driver looking at me? Do they appear distracted (mobile phone, screaming kids, headbanging teens, dog licking drivers face)? Are their wheels starting to roll forward anyway?
I also have a spinning rainbow windsock attached to a flag on our baby trailer. I think the colour, motion, size and wtf factor all contribute to drivers looking and seeing. The kids like it too.
The windsock flag stuck in a backpack has also been good protection from swooping birds, during nesting season magpies and butcher birds are more psycho scary than any driver.
Some recent (night time) research also found drivers differentiate humans from inanimate objects faster when reflectives placed on joints (Google visibility biomotion qut)