The shakespeare-x message: the authorship question solved - Lee Vidor.com - Shakespeare-X Message Source
Apart from that, the argument of this sonnet is similar to that of the previous one: 'Be not wilfully selfish and cruel to mankind, but replace and repair your decaying mansion by procreation. In that way you live on, and I myself and others will think the better of you.'
That = so that;
still = always, continually;
thine = thy children. His beauty will be carried on in his children. beauty here refers to the youth's beauty, both in the individual sense that he as a beautiful youth must preserve himself, but also in that his beauty is the standard for the times, the essential essence of what it is to be beauteous.