I have been attempting to develop languages for my little world. It has been a bit challenging because I am not a trained linguist. Throw in a touch of a learning disability and languages become difficult. In my efforts to develop languages, I draw a lot off of what is already present in our world. Right now, I am active in the effort to use loan words from Anglo Saxon for a group of people and devise words based off of this language.
It has been exceptionally helpful to use the Old English Translator. Between this site and my books about the development of modern English, I have been able to put together a few sentences in old English and insert terms where they'd make sense. In using this site, I have found that precision in your word search is vital. Also, searching out synonyms is a very good idea, because it heightens the likelihood of you locating the one term that you need.
I also have been finding it helpful to use kennings, which are explained very well at Young Poets Network's page here. This practice is a great way to retain some elements of Anglo Saxon wordplay with out having to struggle through the process of finding the right term and the correct way to conjugate it. It was a little awkward at the beginning to create kennings but after a little practice it became fairly easy.
The thing that really has been exceptionally helpful in all of this is reading translations of Anglo Saxon literature. I have been rereading my copy of Beowulf and trying to emulate what I have seen in there. If you are considering picking up a copy of this epic saga, I highly recommend Seamus Heaney's translation. I also recommend looking into other works, such as Cademon's Hymn. Also, consider picking up J.R.R. Tolkien's two volume Book of Lost Tales. His work draws heavily off of his own research into the Anglo Saxon language and can help a modern writer develop something that is similar to the ancient sentence structure and pattern of poetic structure.