to pray in vain, is to pray with selfish ambition.
james 4
submit yourselves to god
1what causes fights and quarrels among you? don't they come from your desires that battle within you? 2you want something but don't get it. you kill and covet, but you cannot have what you want. you quarrel and fight. you do not have, because you do not ask god. 3when you ask, you do not receive, because you ask with wrong motives, that you may spend what you get on your pleasures.

if what you pray for, is what god wills for your life and is good, then your prayers are not in vain.

so, whenever you pray, ask god to first help you pray the way jesus did, cause jesus is our example to follow :)