Praise be to Allah.
The daleel (evidence) for Salaat al-Istikhaarah and the duaa to be recited is the report narrated by al-Bukhaari (1109) and others from Jaabir ibn Abd-Allaah (may Allaah be pleased with them both). For the commentary on this hadeeth and more details, please see question #2217 and 410.
With regard to what some people say, then he should go ahead with whatever he feels happy with, a hadeeth to this effect was narrated from the Prophet (peace and blessings of Allaah be upon him) narrated by Ibn al-Sunni according to which he said, If you are concerned about a matter, then make istikhaarah seven times, praying to your Lord, then see what you feel most drawn towards, and the good will be in that.
Al-Nawawi said: its isnaad is ghareeb, and it includes narrators whom I do not know. (al-Adhkaar, p. 132).
Al-Haafiz ibn Hajar said: if this were proven, it could be taken as evidence, but its isnaad is very weak (waahin jiddan). (al-Fath, 11/223)
Al-Haafiz al-Iraaqi said: it contains a narrator who is known to be daeef jiddan (very weak), namely Ibraaheem ibn al-Baraa.. Therefore the hadeeth is daeef jiddan. (al-Futoohaat al-Rabbaaniyyah, 3/357)
The correct view is that when Allaah makes something easy for you after having decreed it and accepted your duaa this is a sign that it is good to go ahead and do it. The existence of obstacles and difficulties is an indication that Allaah is pushing His slave away from doing it. This meaning will be very clear when one ponders the meaning of the hadeeth of Jaabir concerning al-istikhaarah, where the Prophet (peace and blessings of Allaah be upon him) said:
O Allaah, if You know this affair [and here he should mention it] is good for me in relation to my religion, my life, and end, then decree and facilitate it for me, and bless me with it, and if You know this affair to be ill for me towards my religion, my life, and end, then remove it from me and remove me from it, and decree for me what is good wherever it be and make me satisfied with such.
Ibn Allaan after transmitting the scholarly view that the hadeeth of Anas is daeef (weak) said: Therefore it was said: that after (doing istikhaarah) he should do what he wants (i.e., even if he does not feel that sense of being happy), because whatever happens after doing istikhaarah is good
Al-Haafiz ibn Hajar said: Al-Haafiz Zayn al-Deen al-Iraaqi said (concerning doing something after praying istikhaarah): Whatever he does, there will be good in it. This is supported by what is said at the end of the hadeeth of Ibn Masood according to some of its isnaads: Then let him make his decision. I (Ibn Hajar) say: I have already discussed above and explained that its narrators (i.e., the narrators who added the phrase Then let him make his decision) are daeef (weak). But this is still stronger than the narrator of that hadeeth (i.e., the hadeeth which says, then see what you feel most drawn towards). (al-Futoohaat al-Rabaaniyyah, 3/355-357)
Another of the myths that are widespread among people is that after praying istikhaarah you should sleep, and whatever good you see in your dream that makes you happy means that what you want to do is good and will be made easy for you, otherwise it is not good. (This is what the questioner meant by saying gets the message). There is no saheeh daleel (evidence) for this as far as we know.
The above does not mean that if you feel happy it is not a sign, but this should not be regarded as the only sign about whether or not the matter is good. People often make istikhaarah about something they like or want to do, so they feel happy about it from the outset.
Shaykh al-Islam [Ibn Taymiyah] (may Allaah have mercy on him) said concerning the matter of feeling happy about something: if he makes istakhaarah, then whatever Allaah makes him feel happy about and makes easy for him, this is what Allaah has chosen for him. (Majmoo al-Fataawaa, 10/539).
There is a difference between making feeling happy the only sign and making it one of several signs. There is no set time period for salaat al-istikhaarah; it is permissible to repeat it more than once, but there is no limit to how many times. The person may offer the duaa before or after saying the salaam. And Allaah knows best.