Q: What is the best way to call those who do not pray? What about innovators?.

Firstly:
We should consider the person being called to pray or to do other acts of worship, and pay attention to the most effective means of encouraging him or warning him. The general principle in Islam is to combine both approaches (encouraging and warning). It is also important to pay attention to how receptive or otherwise the person being called is, and whether he will be influences or put off by preaching.
 
Secondly: 
The best way to call those who do not pray can be summed up as follows: 
1 – Reminding them of the obligation of prayer and that it is the greatest of the pillars of Islam after the Shahaadatayn (twin declaration of faith).

2 – Telling them some of the virtues of prayer, for it is the best of the duties that Allaah has enjoined on His slaves, and the best way by means of which a person may draw closer to his Lord. It is the first of his religious affairs for which a person will be brought to account. The five daily prayers are an expiation for whatever sins come in between them, so long as he avoids major sins. A single prostration raises a person in status by one degree, and erases one sin… and there are other virtues of prayer that have been narrated. This may open his heart and perhaps prayer may become a source of delight, as it was for the Prophet (peace and blessings of Allaah be upon him). 
3 – Telling them of the stern warnings that have been narrated concerning not praying, and the different opinions of the scholars as to whether this constitutes kufr (disbelief) and riddah (apostasy). Islam does not give the one who does not pray the opportunity to live freely among people, because in his case he must he invited to pray, and if he persists in not praying, he is to be executed as an apostate according to the view of Ahmad and those who agreed with him among the salaf (early generations of Islam); or he is to be executed as a hadd punishment, according to the view of Maalik and al-Shaafa’i; or he is to be detained and imprisoned, according to the view of Abu Haneefah.  But no scholar says that he is to be left free. So it should be said to the one who does not pray: are you happy that the scholars disagreed as to whether you are a kaafir and whether you should be killed or imprisoned?

4 – Reminding them of the meeting with Allaah, death and the grave, and what will happen to the one who does not pray, namely a bad end and the torment of the grave.
5 – Explaining that delaying prayer until the time for it is over is a major sin:
 “Then, there has succeeded them a posterity who have given up As-Salaah (the prayers) [i.e. made their Salaah (prayers) to be lost, either by not offering them or by not offering them perfectly or by not offering them in their proper fixed times] and have followed lusts. So they will be thrown in Hell”
 [Maryam 19:59 – interpretation of the meaning].
Ibn Mas’ood said concerning al-Ghayy (translated here as “Hell”): it is a valley in Hell that is very deep and has a foul taste.
And Allaah says (interpretation of the meaning):
 “So woe unto those performers of Salaah (prayers) (hypocrites),
5. Those who delay their Salaah (prayer from their stated fixed times)”
 [al-Ma’oon 107:4-5]
6 – Explaining the serious implications of the view that they are kaafirs, such as their marriages being rendered invalid, it being forbidden for them to remain with and be intimate with their wives, and the fact that they will not be washed and prayed for after they die. Among the texts that indicate that the one who does not pray is a kaafir is the hadeeth in which the Prophet (peace and blessings of Allaah be upon him) said: “Between a man and shirk and kufr there stands his giving up prayer.” Narrated by Muslim, 82. and he said: “The covenant that stands between us and them is prayer; whoever gives it up is a kaafir.” Narrated by al-Tirmidhi, 2621; al-Nasaa’i, 463; Ibn Maajah, 1079.

7 – Giving them some booklets and tapes that deal with the subject of prayer and the punishment of the one who does not pray or is careless concerning the prayer.
8 – Forsaking and shunning them if they persist in not praying.
 
With regard to innovators, the way in which we deal with them varies according to the type of innovation involved. We should advise them and call them to Allaah, and establish proof against them, and refute their specious arguments. If the innovator persists in his innovation he should be forsaken and shunned, if it is thought that this will most likely benefit him. We have first to be sure that a person is an innovator before resorting to these measures, and reference should be made to the scholars. We should differentiate between the innovation and the one who practices it, because he may have the excuse of ignorance or misunderstanding. For more details please see Haqeeqat al-Bid’ah wa Ahkaamuhu by Sa’eed ibn Naasir al-Ghaamidi.
And Allaah knows best