Joy is something everyone experiences throughout life–some more than others. Christians who are daily walking in the SPIRIT, being led by the HOLY SPIRIT and abiding in Jesus (John 15.4-5) will have constant joy. That inner joy will be there even during times of trials and persecution. The only way a Christian loses his or her joy is by not abiding in God. So in order to have more joy it is imperative to stay close to the Lord in prayer and Bible study.
When Christians understand the incredible sacrifice Christ made for them, they should always have joy in their hearts for that reason alone, knowing that they have been saved and have an eternal future with Him. (Isaiah 61.10; Romans 15.13; 1 Peter 1.8). When we are in the presence of the Lord (prayer, Bible, fellowship with the brethren) we have joy (Psalm 16.11). We have joy when we study the Scriptures (Jeremiah 15.16; John 15.11), and when we are led and guided by the HOLY SPIRIT (Galatians 5.18, 22).
After having sorrow the Lord gives us joy (Psalm 126.5), and our joy is made full when our prayers are answered (John 16.24). We have joy when we teach the Scriptures (1 John 1.4), and when we learn that those who we disciple are abiding in Jesus (3 John 4). Joy comes from giving to others and sharing the grace of God all to His glory.