What products to avoid..

Ok! Wallaby that was very helpful (BTW I am a huge fan of Lacey and Lady- But then again, who isn't? :p )

For coat: Regular exercise, sweat washed/wiped off after work, good quality feeds and a balanced, healthy nutritional diet, pastured 24/7, correct regular hoof trims, teeth up to date, saddle fitted regularly and tack fitting - and good old fashioned elbow grease. Lots of grooming!

I also rug in cottons in summer (to avoid bleaching).

Nothing beats the true healthy shine that starts from within.

To keep a tail detangled, I only ONLY groom the tail when I wash it (with conditioner), or pick the knots out by hand. Don't brush it daily, it'll cause more breaks. You can plait it, bag it, wrap it, etc.. to keep it cleaner and detangled.
Mane I'm not so worried about, and gets a quick groom before each ride, as both my horses have so much mane. When I start showing, the manes will be kept short and neat.

One again, Keep working. Thanks. . .

Yes, I've had that happen. I tend to avoid all oil based sprays.

Honestly, as long as you're actually moisturizing the mane/tail (like with coconut oil, or something else that actually absorbs) before you use the Show Sheen (or similar product) and wash it out and re-moisturize each time you re-apply, silicones aren't "bad" per say. It's just when people keep applying and applying and applying without adding any moisture to the mix that the hair dries out and gets brittle.

And in fact, by moisturizing and then adding a silicone coating, you're sealing moisture IN, and preventing it from leaching back out of the hair. Just remember that you have to actually add moisture in BEFORE sealing the hair.