Yes, definitely.
You should try to do the impossible to expand your court (vassalize other clans when they are on the brink of destruction, be the plot starter and see others join your faction) so you have an easier chance to marry someone in your own court (very likely to accept). When selecting marriage candidate wives, seek the trait "grey eminence" that gives 30% fertility bonus.
Use your female children to improve relations (give them in betrothal to other clans). I think there's also the chance you may gain land without combat because of marriages (check game guide in forums) but as of the heirs, there's not much you can do.
The fertility decreases with age, check all your wives. Send the old ones to monastery (or kill them with ninja) and try next time you get married to CHECK the traits of the wife. Some traits have good ratings but correspond to bad fertility. There are other traits which increase fertility but I can't remember right now...
