first you need to download APN shortcut app from market,its free. Once install you have option to open up and make new APN which you do with following setting,

Name;Cricket
APN;wap.mycricket.com
Proxy;wap.mycricket.com
Port;8080
UserName;yourcellnumber@mycricket.com
Password;cricket
Server;not set
MMSC;
http://mms.mycricket.com/servlets/mms



MMS proxy;not set
MMS port;not set
MCC;310
MNC;016
Authenitication type;Chap
Apn type; *
APN protocol;ip4/ip6



Once finished press button next to home button and save. reboot phone and then send test mms to yourself,and problem solved! ive been using these settings and have not had mms problem since. Even using mms fix didnt work,but as soon as i made these apn changes problem has never happened again.