this needs to be seen from lagna(ascendant), moon, sun as well as venus.
suppose there is Ketu in 7th house.(dragon's tail)
Ketu's moola mantra is Om Thraam Threem Throum Sa: Ketuvae Nama:
Ketu's devata is Vighneswara or Ganesha.
Ketu's pulse is horse gram.
Ketu's day is tuesday.
Ketu's stars are Aswini, Magha and Moola.
So performing propitiations on these days using the mantras mentioned above, we can make a big difference to Ketu and thus to the 7th house. ie. marriage.
Now, think that Kuja or Mars is the lord of the 7th house.
you need to propitiate Kuja the same as mentioned for Ketu.
Kuja's mantra is Om Kraam Kreem Kroum Sa: Bhoumaya Nama:
Devata is Bhadrakali(in female signs) and Subramania in male signs.
Day is tuesday
Pulse is red gram
Stars are Mrigasirsha, chitra and dhanishta.
likewise for each graha or planet, propitiations are given in my book "understanding and overcoming your darksides" freely available to download in www.jyotishkrishna.com