Hi,
I haven't played Navarre yet, only Moldavia, and just until 1515 or so, takes a while since I'm doing an AAR at the same time. Anyway, what I've noticed so far is:
1. Patience! Wait for the right moment...
2. Timing! Strike when that moment appears!
3. Fortress(es)! To keep you alive while you are elsewhere with your army taking provinces.
4. Diplomacy! Make each dip count (i.e. choose easy targets for alliance).
I don't know what the Navarran income looks like, but as Moldavia, the 'year's end bonus' is pitiful in the start, and the cash you start out with should be used shrewdly. A bailiff should be bought. If possible upgrade the fortress, as this will be crucial in any military endeavours.
Navarre is sadly locked in between two giants, Moldavia at least has Wallachia to take out quickly, but on the other hand the Balkans aren't especially rich and Moldavia has no port. For Navarre, the rich Italian lands are nearby with one- and two-province targets, and you have a fleet, but it could be rather difficult to defend two single and separated provinces...
Anyway, there WILL come a time when you could strike at one neighbour, hopefully with the help of the other, and scrape up some provinces to enable you to get one in a peace settlement. Key: avoid enemy armies, pray your capital's walls will hold, and strike when the enemy's armies are out of sight and they are involved in a major conflict somewhere else.
C. Borgia (1496-1497) is an excellent siege leader, and then you have d'Albret (monarch) until 1515. Use Borgia with the main army for your first offensive as this one is crucial, timing the buildup of your army and your diplomacy for a good setup until he arrives. d'Albret is a perfect 'hanger-on' for an allied army, just follow in their path with him and a small contingent, assault when appropriate and you will receive taken provinces since your leader hold the highest rank.
My 0.02 for the moment... hope it helps a bit!
Good luck!
ph4n_swe