signing up to be cremated after death is a HUGG mistake. Choosing to do soo, im sprry to say is an extremly stupid (sorry I cant think of a nicer way of saying it, but forgive me) thing to do. Why would someone sign up to get scattered all over the universe, I believe this is the form of Kaf Hakella which is worse then Gehinnom (Hell). This is the punishment described for the biggest reshaim like titus (may his name be blotted out).
With tattoes. It is a sin to get them, but that doesn't automatically mean that one is damned to hell forever, expecially if it was forced upon them. Sometimes people look at things as if its some calculated system (sad to say it is the christian and islamic point of view a lot of times). Also they remove the tattoes nowadays after one dies. Having tattoes is a sin and people should be worried, but it doesn't really measure if one is automatically a bad person or not. Getting a tattoe is a sin, but its not every second a sin that your getting while you have it. But it is best to remove it when you can.