God promised to bless Isaac because his father Abraham had followed Him and obeyed His voice. Isaac decided to follow his father's example. He listened to God and stayed in a place called Gerar. We see that despite the famine, Isaac sowed and received a hundred fold because God had blessed him.
There may be times when conditions seem bad. We can't look at how the situation looks, we need to keep on trusting God. He will make a way where there seems to be no way. God always takes care of those who trust Him.
David said in Psalm 27:35 that he has never seen the righteous forsaken, nor his seed begging for bread. God is still the same. He will provide for His children. Regardless of your situation today, trust God in all things. He will work everything out for you.