I used to do this every other week before moving back up.
Advance release is usually 12 weeks in advance - I think there's a page on Virgin's website with dates around which they'll go on sale.
Have a look at split ticketing, sometimes you can save a few quid using, for instance, Euston-Stafford then Stafford-Lime St. tickets on the same train.
If you can be bothered, changing at Crewe onto LNWR or Chester onto Merseyrail is also something to look at.