Biber et al (1999) make this statement: "AmE has an especially strong preference for the pattern verb + bare infinitives although the bare infinitive is more common than the to-infinitive in both varieties."
Longman Grammar of Spoken and Written English, 735
McEnery and Xiao (2005) relate this specifically to the verb "help".
So, according to their paper, "help" is much more prevalent than "help to", especially in American English. The actual percentages they found were, for American English, 79.5% : 20.5%, and for British English, 52.1% : 47.9%. (These percentages have been calculated from the raw figures given in figure 2 of their paper, p.164).
So, for American English, "help improve" is strongly preferred, and for British English you can virtually take your pick!
Biber, D., S. Johansson, G. Leech, S. Conrad, and E. Finegan. Longman Grammar of Spoken and Written English. London: Longman, 1999.
Anthony McEnery and Zhonghua Xiao "HELP or HELP to: What Do Corpora Have to Say?" English Studies. Vol. 86, No. 2, April 2005, pp.161-187