May one say Havdala and do Melacha prior to Maariv? Once Shabbos has ended, which is when a row of three small stars are visible, it is permitted to say Havdala and do Melacha even prior to Davening Maariv. [Nevertheless it is proper to delay doing any Melacha until the congregation has concluded Vayehi Noam, as explained above in 1C. Some Poskim rule it is always proper to first Daven Maariv and then do Havdala in order to follow the set order that the Sages established. ]
Saying Barchu in length: It is accustomed to lengthen the saying of Barchu with a pleasant hymn in order to delay the end of Shabbos and hence add Shabbos into the weekday. [Doing so is a Segula for success and to be saved from injury during the coming week. Likewise it delays the souls from returning to Gehenim.]
May one Daven Maariv early before Shabbos is over: If one will be unable to Daven Maariv and say Havdala after Shabbos, such as he must travel immediately after Shabbos for the purpose of a Mitzvah [or another purpose] he may Daven Maariv and recite Havdala on Shabbos after Plag Hamincha prior to the conclusion of Shabbos. In such a case he does not say a blessing over fire in Havdala, and remains forbidden to do Melacha until a row of three small stars become visible. He likewise must repeat Shema after nightfall. Nevertheless he does not need to repeat Havdala even if the opportunity presents itself after nightfall. If he is traveling for the purpose of a Mitzvah he may choose to only Daven Shemoneh Esrei while it is still Shabbos, and after Shabbos, while traveling, he is to say Shema and its blessings.
Despite the above allowance, some opinions state it is proper for one to refrain from doing the above [i.e. saying Havdala and Maariv while still Shabbos] as it is a puzzling matter to the public.
Are women obligated to Daven Maariv on Motzei Shabbos? Women are not obligated to Daven Maariv on Motzei Shabbos, as is the law regarding Maariv of every night.
Davening Maariv with concentration:
One is to Daven Maariv on Motzaei Shabbos with special concentration, as at that time the Kelipas are reenergized and receive back their power.
Is one who has a Yartzite the coming week to Daven for the Amud on Motzei Shabbos?
Many have the custom to Daven for the Amud on Motzei Shabbos if they have a Yartzite that week. This however is not the Chabad custom, unless of course the Yartzite falls on Sunday.
If one has not yet recited Birchas Hamazon and the Minyan has already begun Maariv, what is he to do?
He is to Daven with the Minyan and recite Birchas Hamazon afterwards. In such a case he does not recite Ritzei in Birchas Hamazon.
The order of the prayer for Maariv on Motzei Shabbos:
- Vehu Rachum and Shir Hamaalos
- Half Kaddish
- Birchas Shema/Shema
- Half Kaddish
- Shemoneh Esrei
- Half Kaddish [Full Kaddish if is week Vayehi Noam is omitted]
- Vayehi Noam [if is week that is said]
- Full Kaddish with Tiskabel
- During Sefira, Sefiras Haomer is now recited.
- Kiddush Levana when applicable
 Seemingly this refers to that even after three consecutive small stars are visible the custom is to delay Maariv even longer, to add even more time to Tosefes Shabbos. [See Kitzur Halachos 293 footnote 5] However Piskeiy Teshuvos 293/1 explains that it is coming to say that one is not to Daven Maariv prior to Tosefes Shabbos, and not that one must add more time past Tosefes Shabbos. See Shaareiy Halacha Uminhag 1/155 that one is not to delay Maariv more than the congregation is able to handle.
 Tehila Ledavid 293/1; see Kitzur Halachos Shabbos p. 119;
In however the Sefer Pear Yisrael 1/207 he writes that the Alter Rebbe at times would say Havdala prior to Maariv.
 Rav Chaim Vital in name of Rav Haiy Gaon. [Kaf Hachaim 293/11]
 Rokeiach brought in Kaf Hachaim 293/12. Based on this one must say there are many levels of returning to Gehenem. [Piskeiy Teshuvos 293 footnote 3
 Admur brings that one has already began walking on Shabbos to the end of the Techum. Vetzaruch Iyun the necessity to say this.
 Kaf Hachaim 293/8
 See Nemukei Orach Chaim 293/1 regarding how this does not contradict the ruling that one may not ask for his bodily needs on Shabbos itself.
 As since he is traveling for a Mitzvah he is allowed to separate Geula from Tefila. [ibid]
However based on Arizal one is to always say Shema before Shemoneh Esrei. [Kaf Hachaim 293/10]
 Bach in name of Rashal brought in Magen Avraham 293/4
 106/2; 299/18
However in 296/18 Admur writes in the first opinion there that according to those which hold Havdala is a Biblical obligation then also women are obligated to say Havdala in Maariv! Perhaps however this means that if they Daven Maariv then according to this opinion they must say Havdala in Maariv, although they are not in truth obligated to Daven Maariv. In any event Admur concludes there that the main opinion is like the opinion which holds Havdala is Rabbinical. See Piskeiy Teshuvos 299/16
 Moreh Betzbah 5/162
 Sefer Haminhagim p. 67
 Beir Moshe 3/53
 See Sefer Haminhagim p. 67; Eliyah Raba 54/4; Shaar Hakolel 17/2; Piskeiy Teshuvos 293/7 for a summary and explanation of the different customs which exist in whether to say Vehu Rachum, or Shir Hamaalos on Motzaei Shabbos.