Most eukaryotic mRNAs contain a short recognition sequence that greatly facilitate the initial binding of mRNA to the small subunit of the ribosome. The consensus sequence for initiation of translation in vertebrates (also called Kozak sequence) is:

ACCATGG

More general it is:

(GCC)RCCATGG

where R is a purine (A or G).

To improve expression levels, it may be advantageous to design the cloned insert according to Kozak's rules.

References:

Kozak, M. Point mutations define a sequence flanking the AUG initiator codon that modulates translation by eukaryotic ribosomes. Cell. 1986, 44(2):283-92.

Kozak, M. An analysis of 5'-noncoding sequences from 699 vertebrate messenger RNAs. Nucleic Acids Res. 1987, 15(20):8125-48

In prokaryotes, the signal for initiation of protein synthesis consists primarily, but not exclusively, of an AUG codon and a rRNA-complementary sequence, the Shine Dalgarno sequence:

AGGAGG

This sequence usually locates 4-7 nucleotides 5' of the initiator AUG of many mRNAs. The sequence is complementary to gaucaCCUCCUuaOH at the 3' end of 16S rRNA.

References:

Shine J, Dalgarno L. Determinant of cistron specificity in bacterial ribosomes. Nature. 1975 Mar 6;254(5495):34-38.