You could always use several layers of encryption, written by different groups
Encrypting something already encrypted has to be done very carefully, otherwise the data is less secure, not more. In the widely known 3DES, which was used as an interim upgrade to DES before AES, the second encryption is actually done with the DES decryption function, while the first and third encryptions are done using the DES encryption function.
And when layering different algorithms, it is possible for the weaknesses of one algorithm to exacerbate the weaknesses of another algorithm. This requires understanding how the algorithms effect each other.
Also, to choose algorithms wisely requires understanding the weaknesses of the algorithms.
The vast majority of potential users of encryption will have to trust more than a few experts and other third parties.