How can I pray better for missionaries?

Get on some missionaries prayer/newsletter list and email lists and pray for them and thier requests as they send them to you.

Get one of their family prayer/photo cards and bring it out at dinner or breakfast and lift them up in prayer at meal times.

One can always pray some Scritpure passages for missionaries, using passages like:

Colossians 1:9-13

Ephesians 1:15-23

Ephesians 3:14-21

Ephesians 6:10-20

Philippians 1:9-11

Acts 4:29-31

2 Thessalonians 3:1-3

Psalm 43:3-5

Psalm 143

Psalm 55

Psalm 56

Psalm 57

See this more in depth article on how to pray for missionaries in a more systematic way.