Ramya I have this problem as well. Since I started shopping more on-line about 4 years ago, I have many times had the problem of purchasing something that looks like it has a normal scoop neckline but when I receive the item it shows everything. My bra strap, my cleavage. The models have a lot more length in their torso and smaller bust lines so I am fooled as to how revealing the top actually is. Now I purchase tops with higher necklines. I have demoted many of these more revealing tops to sleepwear.
However if you have an item you don't want to demote, here are some suggestions.
-Purchase a bra with very deep profile. I have one from Natori and it is perfect for the lowest necklines.
- Alter the straps (especially do-able for tanks and spaghetti strap tops). Shorten them.
- Like you said, wear a cami underneath.