Get on some missionaries prayer/newsletter list and email lists and pray for them and their 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 Scripture 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