The person that clipped my horse during the winter went a bit too close to his mane and took a good strip of mane off with his fur
They made it nice and neat though - they continued with the same line all the way up to his ears!
Now it has grown back it likes to fall forwards, or over to the 'wrong side' and to be honest I quite like it! He is a youngster and quite a cheeky chappy so I think having a punk hair do suits him
My new horse came with a spiky mane (apparently it was long when the seller got him so they pulled it short). It won't lie down at all, even when longer, so I've just pulled it short again. I like him with a short spikey mane, he looks like a punk and it suits his personality!
So to answer the OP, I would leave it unless you *need* it to lie down. If it's long enough it should eventually fall over.
Get the afro shampoo/conditioner range from boots and drench the mane in it, then pull the mane over to the side you like and clip it in place with human hair clips if some pieces like to stick up. It's like getting a side fringe, you have to train the hair to stay in place!
If that doesn't work try the shampoo/conditioner step and then wrap your horses neck with a neck rug or buy some first aid bands, leave them on overnight and the weight should tame the mane down for you!
You really need something to weigh the mane down for you ie, put hair rollers under and above to weigh down the mane but give it a bounce to it