Having Imaan inside our hearts is what makes us Momin. Having a strong Imaan and then doing good deeds are the requirements if we really want to be successful in hereafter.
A Muslim keeps Emaan in Allah, his angels, Prophets, his books, day of Judgement. When a Muslims keeps Emaan in these things then he also takes action in his life according to Guidelines told by Allah and his Prophet SAW.
Level of Emaan not always remains the same but it keeps on changing time to time. it keeps on increasing or decreasing depending upopn the deeds that we do. if we will keep on doing good deeds by avoiding the Haram acts then our Imaan will continue to get stronger. on other hand if we will start to do Haram acts then it will start to decrease with every haram act.
When you feel your Imaan is weaker then you need to recharge it by doing Zikr of Allah SWT as much as possible and by avoiding the haram acts. Offering Salah Regularly, helping the poor, Zikr of Allah, reciting Quran, respect of parents and speaking only good things from your mouth are some acts that will really boost up your Imaan.
Remember the more stronger Emaan you will have the more higher place you will get in Jannah. according to a Hadith there are 70 levels of Emaan. so try to have a stronger Emaan if you want Jannatul firdous in hereafter.