Here's some comparisons between the crop of by-elections this parliament and past parliaments
So unless Miliband completely mucks up he seems to be riding the same wave which Blair rode into a General Election. For the Lib Dems though it looks bleak as they have their lowest by election polling for over 30 years. The added unknown is how much of a difference UKIP will make.
Here is some analysis from by elections to the end of 2012 from the current parliament.