1. At the completion of the du`aa, one should say Aameen.
عن أبي مصبح المقرائى قال كنا نجلس إلى أبى زهير النميرى وكان من الصحابة فيتحدث أحسن الحديث فإذا دعا الرجل منا بدعاء قال اختمه بآمين فإن آمين مثل الطابع على الصحيفة. قال أبو زهير أخبركم عن ذلك خرجنا مع رسول الله صلى الله عليه وسلم ذات ليلة فأتينا على رجل قد ألح فى المسألة فوقف النبى صلى الله عليه وسلم يستمع منه فقال النبى صلى الله عليه وسلم أوجب إن ختم. فقال رجل من القوم بأى شىء يختم قال « بآمين فإنه إن ختم بآمين فقد أوجب ». فانصرف الرجل الذى سأل النبى صلى الله عليه وسلم فأتى الرجل فقال اختم يا فلان بآمين وأبشر (ابو داود رقم 939)
Abu Musabbih Al-Maqraaiy relates: “On one occasion we were seated by Hadhrat Abu Zuhayr An-Numayri (Radhiyallahu Anhu), who was from amongst the Sahaabah (Radhiyallahu Anhum). He was most eloquent in speech. When anyone amongst us used to engage in duaa, he would say, ‘seal the duaa with Aameen, for Aameen is like a stamp on the paper.’ He then said further, ‘should I not inform you regarding this? One night we came out with Rasulullah (Sallallahu Alaihi Wasallam) and came across a person who was fervently engaged in duaa. Rasulullah (Sallallahu Alaihi Wasallam) stopped to listen to him and said: “If he seals his duaa, he will make Jannah binding for him.” One of the people asked, “How should he seal it (his duaa)?” Rasulullah (Sallallahu Alaihi Wasallam) replied, “He should seal it with Ameen, if he seals it with Aameen, he has made Jannah binding for him.” The one who had asked Rasulullah (Sallallahu Alaihi Wasallam) went up to the person and said, “O so and so, seal your duaa with Aameen and accept the glad tidings (of Rasulullah (Sallallahu Alaihi Wasallam)).”
2. After one’s duaa, one should recite durood shareef at the end.
عن عمر بن الخطاب رضي الله عنه قال : إن الدعاء موقوف بين السماء والأرض لا يصعد منه شيء حتى تصلي على نبيك صلى الله عليه و سلم (الترمذي رقم 486)
Hadhrat Umar (Radhiallahu Anhu) narrates, “Duaas remain suspended between the heavens and the Earth. It does not proceed towards the heavens as long as Durood on Nabi (Sallallahu Alaihi Wasallam) has not been recited (i.e. there is no guarantee for its acceptance).