You need to be on an even generation. When i have got them - i have usally have had a teen like a young mimitchi/ichygotchi or hinatchi - and i've looked after it really well by not letting the hearts drop below 1 of each, getting full training, and average weight 40-50 lbs.
There are probably many other ways of getting them too - some people believe you get them by playing 5 games of jumpas a baby, 5 games of bump as a toddler and 5 of heading as a teen - I got one once doing this - but it did have similar care to what i said above as well.
And perhaps this should be in Help for new tamagotchi pet owners?
