Hey,
I tend to print out an a4 sheet or two set up with columns for date lambed, ewe tag no, lamb 1 no. Lamb 2 no. Lamb 3 no. Also the date you turned the ewe and lambs out. And then have a Notes column so that you can add bits in that are relevant like whether you had to assist etc.
The lamb columns I would write M or F in each, circle the M if you rubber ring castrate it and if they died put an X through it so that it's still readable.
I would then at the end of lambing take this home and copy it up onto a spreadsheet or write it up in one of those record books that Zarzar has posted the link to.
Also on turn out I would do as Steve says he does and spray mark the ewe and it's lambs, I would use three different colours example. Blue single, red twins, green triplets then you don't have to carry a piece of paper round remembering what's had what.
Have message you as you can't be too far from me in the hills of Derbyshire.