One should prepare for Selichos by learning (at least one idea from) the Maamar printed at the front of Selichos. Tzedakah should also be given right before Selichos.
It is customary to stand for the entire Selichos. [An infirm person may sit. If possible, he should at least stand for each recitation of Keil Melech Yoishev and Hashem Hashem, as well as Ashamnu.]
Ideally, Ashrei should begin only after a Minyan has gathered. However, if Selichos began without ten people, and a Minyan eventually forms, Chatzi Kaddish may be recited at that time. [The Chazzan should recite several Pesukim beforehand.]
A person who davens without a Minyan should omit the Yud-Gimel Midos-Harachamim. Some maintain that Machei u’Masei and Moron D’vishmaya (end of Selichos) should also be omitted.
If a Chosson is present at Selichos, Ashamnu is omitted. The same applies if one of the Balei Habris are present, provided that it is after dawn.
If there was a Minyan for Selichos but some participants left before the end, Kaddish Tiskabel is still recited, as long as six people still remain. [Obviously, it is forbidden to abandon a Minyan before the end.]
