I've been playing low stakes SNG's (10-50 participants) for a while and learned to beat the game to such a degree that I can play profitably. However, when I attempt playing in MTTs I rarely make it into the money. When the MTT have 500 participants or more it just seems hopeless. What kind of strategy adjustments are advisable compared to the SNG's that I usually play?
I think one of the most important aspects of becoming a good player in any kind of game is knowing yourself. Everyone plays in a different way because they respond to other player's actions and to the cards they're dealt in a different way, because that's just how they are.
I suffer from the opposite problem of yours. I usually make into the money in 500~5000 players tournaments - which turns out to be really profitable - however, I often lose at small SNGs and am not-so-good not-so-bad at final tables. I found out that, although I'm certainly not a tight player, I tend to avoid risky hands and big decisions. I play what is called smallball poker, which is actually very common in MTTs, but not as much in small SNGs - because it doesn't work that well in it, specially the so called turbo ones, where the blinds go up real quickly and you have to make big decisions. So I had to adjust my strategy to play SNGs. I personally think smallball poker is a very good strategy for long, big MTTs, and it has been working pretty well for me, but that's because it fits my personality.
Players like Gus Hansen made their way to the top by playing very aggressively all through a lot of tournaments. There was this guy - which I can't remember the name - that came from limit poker and made a fortune playing very tightly on MTTs.
What I'm trying to say is: It doesn't really matter what strategy you use. Some usually work better than others in this or that type of game, but if it doesn't fit you, it might work sometimes, but it'll usually fail. Find out what's your kind of game and use a strategy that suits you the best - I listed some as suggestions.
The biggest difference in the 50 people sng and the 500 mtt is just that. Do not make any adjustments if you feel you are profitable. Remember though if you are itm 20% in this SNGs which is 1/5 times, you can now only expect to be itm 1/50 times due to the sheer volume of players. This means it takes you longer to approach that EV that you deserve. The other big difference is the amount of time these tournaments take. Make sure you feel fresh, because 5 hours in, it is easy to be less objective and pull off some silly moves that you would not otherwise.