1) They overcame him (the accuser of the brethren, the devil) by the blood of the Lamb - by being washed in Jesus' blood which He shed at Calvary for their sins (Matthew 26:28; Ephesians 1:7; Revelation 1:5), and by being cleansed by His blood on a daily basis (1 John 1:7, 9).
2) And by the word of their testimony - These Christians were victorious because they were witnesses, they shared their testimonies with others! On the other side of the coin, this verse indicates that if we will not witness about the Lord Jesus Christ and the eternal salvation found only in Him, we will not be overcomers! (Note: this is referring to overcoming on a day by day basis - though born again believers are more than conquerors in Christ - we are overcomers through faith in Him according to Revelation 2-3 and 1 John 5:4-5 - we do not always walk in light of that victory. This is a key passage for showing us how we can obtain that victory on a daily basis, as we walk in right fellowship with the Lord.)
3) And they loved not their lives unto the death - They were more concerned with pleasing God than they were about being menpleasers; they were more concerned with being obedient and faithful to the Lord than they were about compromising to keep their lives. They would rather die than turn away from Jesus Christ, rather die than betray their Saviour. (See Matthew 16:24-26; Colossians 3:22-24) Their integrity and spiritual well-being was more important to them than their comfort and physical well-being. (See Daniel 3:16-18)