There are millions of alternative training programs/ routines etc
Are you getting good results from the training you are doing at the moment? I.e. are you seeing improvements in your press-ups/ sit-ups scores?
If it is working and you are seeing good progress, then there is not neccessarily any need to change things around. Make sure you aren't just banging out the same number of reps week in week out, you want to be progressing.
You may want to mix it up if you are getting bored, or so that your muscles don't get "used" to set routines. Then there are loads of variations you can do if you use your imagination or have a look through some of the posts on here.
atb