Vanguard Mutual Fund query

I put 4k into VTSAX in January and it is currently up $249 (4,249). It was up $150 in the first week I purchased and has dipped lower and higher since then but generally hovered in the +-200~ range week to week. Should I expect it to eventually trend higher or do I have to put more cash in to see larger gains? Just unsure how to lock gains in with a fund like this. Thanks!