Most people have fiqured out that in Onslaught mode, pressing the Bio Rifle key (the "SwitchWeapon 3" command, bound to the 3 key by default) will select the Spider Mine Layer or the Bio Rifle, depending on which weapon you have. If you have both the Bio Rifle and the Spider Mine Layer, every time you press the key it will switch between the two weapons.
However, if you don't mind manually editing your user.ini file, you can setup keys to directly select the Spider Mine Layer. The following command will select the Spider Mine Layer:
GetWeapon Onslaught.ONSMineLayer
You can assign these directly to keys as in:
E=GetWeapon Onslaught.ONSMineLayer
or you can assign it to an alias and assign the alias to a key, as in:
Aliases[32]=(Command="GetWeapon Onslaught.ONSMineLayer",Alias="GetMine") ..Further down... E=GetMine
The best place to use the mine layer is in choke points, preferable off to the side where they are partially or completely hidden from view until an unsuspecting vehicle (or player on foot :)) stumbles into the mines.
I also fire a few Mines onto the Manta before I jump in; usually 2 or 3, because any more than that are hard to place quickly. (They will stick if you place them correctly). Then, if a player on foot dodges me when I try to Pancake them, the spider mines attack! :-D
(By the way, if you're planning on using mines against vehicles, stick some grenades to them to increase their damage capacity. - Exploding Tomato)