Drilling perpendicular (straight) holes

[George Yetka]

I dont think this is extremely necessary. But anytime I see someone on youtube drilling a hole it usually doesnt appear straight.

[Thomas McCurnin]

+1 on Edward's method. I make a thick block of wood out of scrap, use a long drill bit to create a hole, and sometimes add a fence to the underside. It's a one-off jig.

For more permanent solutions, I drill the hole oversize in relation to the drill bit size, and insert a drill bushing for the drill bit size. When I'm done, I knock out the bushing to re-use it later.

[Edward Weber]

One-off jig is what they start out as but they usually hang around for a while.
I've got several for different purposes, (with guide fences) for cabinet knob location, leveling feet location and so on.