Most modern cars have reverse sensors in them, and many have reversing cameras. These conveniences help a lot in ensuring that you don't hit the car behind you when reverse parking.
But I find it very difficult in estimating the distance to the car in front of me when parallel parking. I often find myself being too conservative when parking in a small spot, because I think I'm 10cm away from the car in front of me but I'm actually 50cm away. So I get out of the car, realise I have lots of more room, jump back in and edge forward a bit [repeat until satisfied].
If there's a window or reflective surface nearby this is quite easy, but these aren't always available. Any hints about how to make this easier?