This thread at the Ubuntu forums seems usefull as: This HowTo is for those who complaint ubuntu boot-up speed is pretty slow but not willing to install any alternative tools to speed up.
Read the guide here
The way I use here is not the altimate solution by any means but it does make differences and it does work. Everything done below is by tuning the boot process itself and because everyone's computer might be different, there is a little risk that something below might break your system. Take your own judgment before you perform a change and always good to do a backup for the /etc dir.